Red Hat Training

A Red Hat training course is available for RHEL 8

Configuring device mapper multipath

Red Hat Enterprise Linux 8

Using the Device Mapper Multipath feature

Red Hat Customer Content Services

Abstract

This documentation collection provides instructions on how to configure and manage the Device Mapper Multipath (DM-Multipath) feature on Red Hat Enterprise Linux 8.

Making open source more inclusive

Red Hat is committed to replacing problematic language in our code, documentation, and web properties. We are beginning with these four terms: master, slave, blacklist, and whitelist. Because of the enormity of this endeavor, these changes will be implemented gradually over several upcoming releases. For more details, see our CTO Chris Wright’s message.

Providing feedback on Red Hat documentation

We appreciate your input on our documentation. Please let us know how we could make it better. To do so:

  • For simple comments on specific passages:

    1. Make sure you are viewing the documentation in the Multi-page HTML format. In addition, ensure you see the Feedback button in the upper right corner of the document.
    2. Use your mouse cursor to highlight the part of text that you want to comment on.
    3. Click the Add Feedback pop-up that appears below the highlighted text.
    4. Follow the displayed instructions.
  • For submitting more complex feedback, create a Bugzilla ticket:

    1. Go to the Bugzilla website.
    2. As the Component, use Documentation.
    3. Fill in the Description field with your suggestion for improvement. Include a link to the relevant part(s) of documentation.
    4. Click Submit Bug.

Chapter 1. Overview of Device Mapper Multipathing

With Device mapper multipathing (DM Multipath), you can configure multiple I/O paths between server nodes and storage arrays into a single device. These I/O paths are physical Storage Area Network (SAN) connections that can include separate cables, switches, and controllers. Multipathing aggregates the I/O paths and creates a new device that consists of the aggregated paths.

DM Multipath provides:

  • Redundancy

    DM Multipath can provide failover in an active/passive configuration. In an active/passive configuration, only half the paths are used at any time for I/O. If any element of an I/O path (the cable, switch, or controller) fails, DM Multipath switches to an alternate path.

  • Improved Performance

    DM Multipath can be configured in an active/active mode, where I/O is spread over the paths in a round-robin fashion. In some configurations, DM Multipath can detect loading on the I/O paths and dynamically rebalance the load.

1.1. Different DM Multipath configurations

Following are the few DM Multipath configuration examples:

1.1.1. Active/Passive multipath configuration with one RAID device

In this configuration, there are two Host Bus Adapters (HBAs) on the server, two SAN switches, and two RAID controllers. Following are the possible failure in this configuration:

  • HBA failure
  • Fibre Channel cable failure
  • SAN switch failure
  • Array controller port failure

With DM Multipath configured, a failure at any of these points causes DM Multipath to switch to the alternate I/O path. Figure 1.1, “Active/Passive multipath configuration with one RAID device” describes the configuration with two I/O paths from the server to a RAID device. Here, there is one I/O path that goes through hba1, SAN1, and cntrlr1 and a second I/O path that goes through hba2, SAN2, and cntrlr2.

Figure 1.1. Active/Passive multipath configuration with one RAID device

active passive multipath configuration with one raid device

1.1.2. Active/Passive multipath configuration with two RAID devices

In this configuration, there are two HBAs on the server, two SAN switches, and two RAID devices with two RAID controllers each. With DM Multipath configured, a failure at any of the points of the I/O path to either of the RAID devices causes DM Multipath to switch to the alternate I/O path for that device. Figure 1.2, “Active/Passive multipath configuration with two RAID device” describes the configuration with two I/O paths to each RAID device. Here, there are two I/O paths to each RAID device.

Figure 1.2. Active/Passive multipath configuration with two RAID device

active passive multipath configuration with two raid device

1.1.3. Active/Active multipath configuration with one RAID device

In this configuration, there are two HBAs on the server, two SAN switch, and two RAID controllers. Figure 1.3, “Active/Active multipath configuration with one RAID device” describes the configuration with two I/O paths from the server to a storage device. Here, I/O can be spread among these two paths.

Figure 1.3. Active/Active multipath configuration with one RAID device

active passive multipath configuration with one raid device

1.2. DM Multipath components

Table 1.1, “Components of DM Multipath” describes the DM Multipath components.

Table 1.1. Components of DM Multipath

Component

Description

dm_multipath kernel module

Reroutes I/O and supports failover for paths and path groups.

mpathconf utility

Configures and enables device mapper multipathing.

multipath command

Lists and configures the multipath devices. It is also executed by udev whenever a block device is added, to determine if the device should be part of a multipath device or not.

multipathd daemon

Automatically creates and removes multipath devices and monitors paths; as paths fail and come back, it may update the multipath device. Allows interactive changes to multipath devices. Reload the service if there are any changes to the /etc/multipath.conf file.

kpartx command

Creates device mapper devices for the partitions on a device. This command is automatically executed by udev when multipath devices are created to create partition devices on top of them. The kpartx command is provided in its own package, but the device-mapper-multipath package depends on it.

mpathpersist

Sets up SCSI-3 persistent reservations on multipath devices. This command works similarly to the way sg_persist works for SCSI devices that are not multipathed, but it handles setting persistent reservations on all paths of a multipath device. It coordinates with multipathd to ensure that the reservations are set up correctly on paths that are added later. To use this functionality, the reservation_key attribute must be defined in the /etc/multipath.conf file. Otherwise the multipathd daemon will not check for persistent reservations for newly discovered paths or reinstated paths.

Additional resources

  • The multipath man page.
  • The multipathd man page.
  • The /etc/multipath.conf file.

Chapter 2. Multipath devices

Without DM Multipath, system treats each path from a server node to a storage controller as a separate device, even when the I/O path connects the same server node to the same storage controller. DM Multipath provides a way of organizing the I/O paths logically, by creating a single multipath device on top of the underlying devices.

2.1. Multipath device identifiers

When new devices are under the control of DM Multipath, these devices are created in the /dev/mapper/ and /dev/ directory. Any devices of the form /dev/dm-X are for internal use only and should never be used by the administrator directly.

Multipath device names:

  • When the user_friendly_names configuration option is set to no, the name of the multipath device is set to World Wide Identifier (WWID). By default, the name of a multipath device is set to its WWID. The device name would be /dev/mapper/WWID. It is also created in the /dev/ directory, named as /dev/dm-X.
  • Alternately, you can set the user_friendly_names option to yes in the /etc/multipath.conf file. This sets the alias in the multipath section to a node-unique name of the form mpathN. The device name would be /dev/mapper/mpathN and /dev/dm-X. But the device name is not guaranteed to be the same on all nodes using the multipath device. Similarly, if you set the alias option in the /etc/multipath.conf file, the name is not automatically consistent across all nodes in the cluster.

    This should not cause any difficulties if you use LVM to create logical devices from the multipath device. To keep your multipath device names consistent in every node, Red Hat recommends to disable the user_friendly_names option.

For example, a node with two HBAs attached to a storage controller with two ports by means of a single unzoned FC switch sees four devices: /dev/sda, /dev/sdb, /dev/sdc, and /dev/sdd. DM Multipath creates a single device with a unique WWID that reroutes I/O to those four underlying devices according to the multipath configuration.

In addition to the user_friendly_names and alias options, a multipath device also has other attributes. You can modify these attributes for a specific multipath device by creating an entry for that device in the multipaths section of the /etc/multipath.conf file.

Additional resources

2.2. Multipath devices in logical volumes

After creating multipath devices, you can use the multipath device names just as you would use a physical device name when creating an LVM physical volume. For example, if /dev/mapper/mpatha is the name of a multipath device, the pvcreate /dev/mapper/mpatha command marks /dev/mapper/mpatha as a physical volume.

You can use the resulting LVM physical device when you create an LVM volume group just as you would use any other LVM physical device.

Note

If you attempt to create an LVM physical volume on a whole device on which you have configured partitions, the pvcreate command fails. The Anaconda and Kickstart installation programs create empty partition tables if you do not specify otherwise for every block device. If you want to use the whole device instead of creating a partition, remove the existing partitions from the device. You can remove existing partitions with the kpartx -d device command and the fdisk utility. If your system has block devices that are greater than 2Tb, use the parted utility to remove partitions.

When you create an LVM logical volume that uses active/passive multipath arrays as the underlying physical devices, you can optionally, include filters in the /etc/lvm/lvm.conf file to exclude the disks that underline the multipath devices. This is because if the array automatically changes the active path to the passive path when it receives I/O, multipath will failover and failback whenever LVM scans the passive path if these devices are not filtered. For active/passive arrays that require a command to make the passive path active, LVM prints a warning message when this occurs.

To filter all the sd devices in the /etc/lvm/lvm.conf file, add the filter = [ "r/block/", "r/disk/", "r/sd./", "a/./" ] filter in the devices section of the file.

Additional resources

Chapter 3. Setting up DM Multipath

Before setting up DM Multipath on your system, ensure that your system has been updated and includes the device-mapper-multipath package.

3.1. Basic DM Multipath setup

You set up DM Multipath with the mpathconf utility, which creates the multipath configuration file /etc/multipath.conf.

  • If the /etc/multipath.conf file already exists, the mpathconf utility will edit it.
  • If the /etc/multipath.conf file does not exist, the mpathconf utility will create the /etc/multipath.conf file from scratch.

For more information on the mpathconf utility, see the mpathconf(8) man page.

If you do not need to edit the /etc/multipath.conf file, you can set up DM Multipath for a basic failover configuration by running the following mpathconf command. This command enables the multipath configuration file and starts the multipathd daemon.

# mpathconf --enable --with_multipathd y

If you need to edit the /etc/multipath.conf file before starting the multipathd daemon, use the following procedure to set up DM Multipath for a basic failover configuration.

  1. Enter the mpathconf command with the --enable option specified:

    # mpathconf --enable

    For information on additional options to the mpathconf command you may require, see the mpathconf(8) man page or enter the mpathconf command with the --help option specified.

    # mpathconf --help
    usage: /sbin/mpathconf <command>
    
    Commands:
    Enable: --enable
    Disable: --disable
    Set user_friendly_names (Default y): --user_friendly_names <y|n>
    Set find_multipaths (Default y): --find_multipaths <y|n>
    Load the dm-multipath modules on enable (Default y): --with_module  <y|n>
    start/stop/reload multipathd (Default n): --with_multipathd  <y|n>
  2. Edit the /etc/multipath.conf file if necessary. The default settings for DM Multipath are compiled in to the system and do not need to be explicitly set in the /etc/multipath.conf file.

    The default value of path_grouping_policy is set to failover, so in this example you do not need to edit the /etc/multipath.conf file.

    The initial defaults section of the configuration file configures your system so that the names of the multipath devices are of the form /dev/mapper/mpathn; without this setting, the names of the multipath devices would be aliased to the WWID of the device. If you do not want to use user friendly names, you can enter the following command:

    # mpathconf --enable --user_friendly_names n
  3. Save the configuration file and exit the editor, if necessary.
  4. Execute the following command:

    # systemctl start multipathd.service
Note

If you find that you need to edit the multipath configuration file after you have started the multipath daemon, you must execute the systemctl reload multipathd.service command for the changes to take effect.

3.2. Ignoring local disks when generating multipath devices

Some machines have local SCSI cards for their internal disks. DM Multipath is not recommended for these devices. If you set the find_multipaths configuration parameter to on, you do not have to disable multipathing on these devices. If you do not set the find_multipaths configuration parameter to on, you can use the following procedure to modify the DM Multipath configuration file to ignore the local disks when configuring multipath.

Procedure

  1. Determine which disks are the internal disks. In these examples, /dev/sda is the internal disk:

    • Display existing multipath devices:

      # multipath -v2 -l
      
      SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1 dm-2 WINSYS,SF2372
      size=33 GB features="0" hwhandler="0" wp=rw
      `-+- policy='round-robin 0' prio=0 status=active
        |- 0:0:0:0 sda 8:0 active undef running
    • Display additional multipath devices that DM Multipath could create:

      # multipath -v2 -d
      
      : SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1 undef WINSYS,SF2372
      size=33 GB features="0" hwhandler="0" wp=undef
      `-+- policy='round-robin 0' prio=1 status=undef
        |- 0:0:0:0 sda 8:0  undef ready running
  2. Edit the blacklist section of the /etc/multipath.conf file to include this device.

    Identify the device using its WWID attribute. Although you could identify the sda device using a devnode type, that would not be a safe procedure because /dev/sda is not guaranteed to be the same on reboot.

    In the previous example, the WWID of the /dev/sda device is SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1. To ignore this device, include the following in the /etc/multipath.conf file:

    blacklist {
          wwid SIBM-ESXSST336732LC____F3ET0EP0Q000072428BX1
    }
  3. Reload the updated /etc/multipath.conf file:

    # systemctl reload multipathd.service

Additional resources

  • For information on the find_multipaths configuration parameter and the meaning of the values to which you can set this parameter to, see the multipath.conf(5) man page.

3.3. Configuring additional storage devices

By default, DM Multipath includes support for the most common storage arrays that themselves support DM Multipath. For information on the default configuration value, including supported devices, run either of the following commands.

# multipathd show config
# multipath -t

If you need to add a storage device that is not supported by default as a known multipath device, edit the /etc/multipath.conf file and insert the appropriate device information.

For example, to add information about the HP Open-V series the entry looks like this. This example sets the device to queue for a minute (or 12 retries and 5 seconds per retry) after all paths have failed.

devices {
        device {
                vendor "HP"
                product "OPEN-V"
                no_path_retry 12
        }
}

3.4. Setting up multipathing in the initramfs file system

You can set up multipathing in the initramfs file system. After configuring multipath, you can rebuild the initramfs file system with the multipath configuration files by executing the dracut command with the following options:

# dracut --force --add multipath

If you run multipath from the initramfs file system and you make any changes to the multipath configuration files, you must rebuild the initramfs file system for the changes to take effect.

Chapter 4. Enabling multipathing on NVMe devices

You can multipath NVMe devices that are connected to your system over a fabric transport, such as Fibre Channel (FC). You can select between multiple multipathing solutions.

4.1. Native NVMe multipathing and DM Multipath

NVMe devices support a native multipathing functionality. When configuring multipathing on NVMe, you can select between the standard DM Multipath framework and the native NVMe multipathing.

Both DM Multipath and native NVMe multipathing support the Asymmetric Namespace Access (ANA) multipathing scheme of NVMe devices. ANA identifies optimized paths between the target and the initiator and improves performance.

When native NVMe multipathing is enabled, it applies globally to all NVMe devices. It can provide higher performance, but does not contain all of the functionality that DM Multipath provides. For example, native NVMe multipathing supports only the failover and round-robin path selection methods.

Red Hat recommends that you use DM Multipath in RHEL 8 as your default multipathing solution.

4.2. Enabling native NVMe multipathing

This procedure enables multipathing on connected NVMe devices using the native NVMe multipathing solution.

Prerequisites

Procedure

  1. Check if native NVMe multipathing is enabled in the kernel:

    # cat /sys/module/nvme_core/parameters/multipath

    The command displays one of the following:

    N
    Native NVMe multipathing is disabled.
    Y
    Native NVMe multipathing is enabled.
  2. If native NVMe multipathing is disabled, enable it using one of the following methods:

    • Using a kernel option:

      1. Add the nvme_core.multipath=Y option on the kernel command line:

        # grubby --update-kernel=ALL --args="nvme_core.multipath=Y"
      2. On the 64-bit IBM Z architecture, update the boot menu:

        # zipl
      3. Reboot the system.
    • Using a kernel module configuration file:

      1. Create the /etc/modprobe.d/nvme_core.conf configuration file with the following content:

        options nvme_core multipath=Y
      2. Back up the initramfs file system:

        # cp /boot/initramfs-$(uname -r).img \
             /boot/initramfs-$(uname -r).bak.$(date +%m-%d-%H%M%S).img
      3. Rebuild the initramfs file system:

        # dracut --force --verbose
      4. Reboot the system.
  3. Optional: On the running system, change the I/O policy on NVMe devices to distribute the I/O on all available paths:

    # echo "round-robin" > /sys/class/nvme-subsystem/nvme-subsys0/iopolicy
  4. Optional: Set the I/O policy persistently using udev rules. Create the /etc/udev/rules.d/71-nvme-io-policy.rules file with the following content:

    ACTION=="add|change", SUBSYSTEM=="nvme-subsystem", ATTR{iopolicy}="round-robin"

Verification

  1. Check that your system recognizes the NVMe devices:

    # nvme list
    
    Node             SN                   Model                                    Namespace Usage                      Format           FW Rev
    ---------------- -------------------- ---------------------------------------- --------- -------------------------- ---------------- --------
    /dev/nvme0n1     a34c4f3a0d6f5cec     Linux                                    1         250.06  GB / 250.06  GB    512   B +  0 B   4.18.0-2
    /dev/nvme0n2     a34c4f3a0d6f5cec     Linux                                    2         250.06  GB / 250.06  GB    512   B +  0 B   4.18.0-2
  2. List all connected NVMe subsystems:

    # nvme list-subsys
    
    nvme-subsys0 - NQN=testnqn
    \
     +- nvme0 fc traddr=nn-0x20000090fadd597a:pn-0x10000090fadd597a host_traddr=nn-0x20000090fac7e1dd:pn-0x10000090fac7e1dd live
     +- nvme1 fc traddr=nn-0x20000090fadd5979:pn-0x10000090fadd5979 host_traddr=nn-0x20000090fac7e1dd:pn-0x10000090fac7e1dd live
     +- nvme2 fc traddr=nn-0x20000090fadd5979:pn-0x10000090fadd5979 host_traddr=nn-0x20000090fac7e1de:pn-0x10000090fac7e1de live
     +- nvme3 fc traddr=nn-0x20000090fadd597a:pn-0x10000090fadd597a host_traddr=nn-0x20000090fac7e1de:pn-0x10000090fac7e1de live

    Check the active transport type. For example, nvme0 fc indicates that the device is connected over the Fibre Channel transport, and nvme tcp indicates that the device is connected over TCP.

  3. If you edited the kernel options, check that native NVMe multipathing is enabled on the kernel command line:

    # cat /proc/cmdline
    
    BOOT_IMAGE=[...] nvme_core.multipath=Y
  4. Check that DM Multipath reports the NVMe namespaces as, for example, nvme0c0n1 through nvme0c3n1, and not as, for example, nvme0n1 through nvme3n1:

    # multipath -ll | grep -i nvme
    
    uuid.8ef20f70-f7d3-4f67-8d84-1bb16b2bfe03 [nvme]:nvme0n1 NVMe,Linux,4.18.0-2
    | `- 0:0:1    nvme0c0n1 0:0     n/a   optimized live
    | `- 0:1:1    nvme0c1n1 0:0     n/a   optimized live
    | `- 0:2:1    nvme0c2n1 0:0     n/a   optimized live
      `- 0:3:1    nvme0c3n1 0:0     n/a   optimized live
    
    uuid.44c782b4-4e72-4d9e-bc39-c7be0a409f22 [nvme]:nvme0n2 NVMe,Linux,4.18.0-2
    | `- 0:0:1    nvme0c0n1 0:0     n/a   optimized live
    | `- 0:1:1    nvme0c1n1 0:0     n/a   optimized live
    | `- 0:2:1    nvme0c2n1 0:0     n/a   optimized live
      `- 0:3:1    nvme0c3n1 0:0     n/a   optimized live
  5. If you changed the I/O policy, check that round-robin is the active I/O policy on NVMe devices:

    # cat /sys/class/nvme-subsystem/nvme-subsys0/iopolicy
    
    round-robin

Additional resources

4.3. Enabling DM Multipath on NVMe devices

This procedure enables multipathing on connected NVMe devices using the DM Multipath solution.

Prerequisites

Procedure

  1. Check that native NVMe multipathing is disabled:

    # cat /sys/module/nvme_core/parameters/multipath

    The command displays one of the following:

    N
    Native NVMe multipathing is disabled.
    Y
    Native NVMe multipathing is enabled.
  2. If native NVMe multipathing is enabled, disable it:

    1. Remove the nvme_core.multipath=Y option from the kernel command line:

      # grubby --update-kernel=ALL --remove-args="nvme_core.multipath=Y"
    2. On the 64-bit IBM Z architecture, update the boot menu:

      # zipl
    3. Remove the options nvme_core multipath=Y line from the /etc/modprobe.d/nvme_core.conf file, if it is present.
    4. Reboot the system.
  3. Make sure that DM Multipath is enabled:

    # systemctl enable --now multipathd.service
  4. Distribute I/O on all available paths. Add the following content in the /etc/multipath.conf file:

    device {
      vendor "NVME"
      product ".*"
      path_grouping_policy    group_by_prio
    }
    Note

    The /sys/class/nvme-subsystem/nvme-subsys0/iopolicy configuration file has no effect on the I/O distribution when DM Multipath manages the NVMe devices.

  5. Reload the multipathd service to apply the configuration changes:

    # multipath -r
  6. Back up the initramfs file system:

    # cp /boot/initramfs-$(uname -r).img \
         /boot/initramfs-$(uname -r).bak.$(date +%m-%d-%H%M%S).img
  7. Rebuild the initramfs file system:

    # dracut --force --verbose

Verification

  1. Check that your system recognizes the NVMe devices:

    # nvme list
    
    Node             SN                   Model                                    Namespace Usage                      Format           FW Rev
    ---------------- -------------------- ---------------------------------------- --------- -------------------------- ---------------- --------
    /dev/nvme0n1     a34c4f3a0d6f5cec     Linux                                    1         250.06  GB / 250.06  GB    512   B +  0 B   4.18.0-2
    /dev/nvme0n2     a34c4f3a0d6f5cec     Linux                                    2         250.06  GB / 250.06  GB    512   B +  0 B   4.18.0-2
    /dev/nvme1n1     a34c4f3a0d6f5cec     Linux                                    1         250.06  GB / 250.06  GB    512   B +  0 B   4.18.0-2
    /dev/nvme1n2     a34c4f3a0d6f5cec     Linux                                    2         250.06  GB / 250.06  GB    512   B +  0 B   4.18.0-2
    /dev/nvme2n1     a34c4f3a0d6f5cec     Linux                                    1         250.06  GB / 250.06  GB    512   B +  0 B   4.18.0-2
    /dev/nvme2n2     a34c4f3a0d6f5cec     Linux                                    2         250.06  GB / 250.06  GB    512   B +  0 B   4.18.0-2
    /dev/nvme3n1     a34c4f3a0d6f5cec     Linux                                    1         250.06  GB / 250.06  GB    512   B +  0 B   4.18.0-2
    /dev/nvme3n2     a34c4f3a0d6f5cec     Linux                                    2         250.06  GB / 250.06  GB    512   B +  0 B   4.18.0-2
  2. List all connected NVMe subsystems. Check that the command reports them as, for example, nvme0n1 through nvme3n2, and not as, for example, nvme0c0n1 through nvme0c3n1:

    # nvme list-subsys
    
    nvme-subsys0 - NQN=testnqn
    \
     +- nvme0 fc traddr=nn-0x20000090fadd5979:pn-0x10000090fadd5979 host_traddr=nn-0x20000090fac7e1dd:pn-0x10000090fac7e1dd live
     +- nvme1 fc traddr=nn-0x20000090fadd597a:pn-0x10000090fadd597a host_traddr=nn-0x20000090fac7e1dd:pn-0x10000090fac7e1dd live
     +- nvme2 fc traddr=nn-0x20000090fadd5979:pn-0x10000090fadd5979 host_traddr=nn-0x20000090fac7e1de:pn-0x10000090fac7e1de live
     +- nvme3 fc traddr=nn-0x20000090fadd597a:pn-0x10000090fadd597a host_traddr=nn-0x20000090fac7e1de:pn-0x10000090fac7e1de live
    # multipath -ll
    
    mpathae (uuid.8ef20f70-f7d3-4f67-8d84-1bb16b2bfe03) dm-36 NVME,Linux
    size=233G features='1 queue_if_no_path' hwhandler='0' wp=rw
    `-+- policy='service-time 0' prio=50 status=active
      |- 0:1:1:1  nvme0n1 259:0   active ready running
      |- 1:2:1:1  nvme1n1 259:2   active ready running
      |- 2:3:1:1  nvme2n1 259:4   active ready running
      `- 3:4:1:1  nvme3n1 259:6   active ready running
    
    mpathaf (uuid.44c782b4-4e72-4d9e-bc39-c7be0a409f22) dm-39 NVME,Linux
    size=233G features='1 queue_if_no_path' hwhandler='0' wp=rw
    `-+- policy='service-time 0' prio=50 status=active
      |- 0:1:2:2  nvme0n2 259:1   active ready running
      |- 1:2:2:2  nvme1n2 259:3   active ready running
      |- 2:3:2:2  nvme2n2 259:5   active ready running
      `- 3:4:2:2  nvme3n2 259:7   active ready running

Additional resources

Chapter 5. Modifying the DM-Multipath configuration file

By default, DM Multipath provides configuration values for the most common uses of multipathing. In addition, DM Multipath includes support for the most common storage arrays that themselves support DM Multipath. You can override the default configuration values for DM Multipath by editing the /etc/multipath.conf configuration file. If necessary, you can also add a storage array that is not supported by default to the configuration file.

For information on the default configuration values, including supported devices, run either of the following commands:

# multipathd show config
# multipath -t
Note

You can run set up multipathing in the initramfs file system. If you run multipath from the initramfs file system and you make any changes to the multipath configuration files, you must rebuild the initramfs file system for the changes to take effect.

In the multipath configuration file, you need to specify only the sections that you need for your configuration, or that you wish to change from the default values. If there are sections of the file that are not relevant to your environment or for which you do not need to override the default values, you can leave them commented out, as they are in the initial file.

The configuration file allows regular expression description syntax.

5.1. Configuration file overview

The multipath configuration file is divided into the following sections:

blacklist
Listing of specific devices that will not be considered for multipath.
blacklist_exceptions
Listing of multipath candidates that would otherwise be ignored according to the parameters of the blacklist section.
defaults
General default settings for DM Multipath.
multipaths
Settings for the characteristics of individual multipath devices. These values overwrite what is specified in the overrides, devices, and defaults sections of the configuration file.
devices
Settings for the individual storage controllers. These values overwrite what is specified in the defaults section of the configuration file. If you are using a storage array that is not supported by default, you may need to create a devices subsection for your array.
overrides
Settings that are applied to all devices. These values overwrite what is specified in the devices and defaults sections of the configuration file.

When the system determines the attributes of a multipath device, first it checks the multipath settings, then the devices settings, then the multipath system defaults.

5.2. DM Multipath overrides of the device timeout

The recovery_tmo sysfs option controls the timeout for a particular iSCSI device. The following options globally override recovery_tmo values:

  • The replacement_timeout configuration option globally overrides the recovery_tmo value for all iSCSI devices.
  • For all iSCSI devices that are managed by DM Multipath, the fast_io_fail_tmo option in DM Multipath globally overrides the recovery_tmo value.

    The fast_io_fail_tmo option in DM Multipath also overrides the fast_io_fail_tmo option in Fibre Channel devices.

The DM Multipath fast_io_fail_tmo option takes precedence over replacement_timeout. Red Hat does not recommend using replacement_timeout to override recovery_tmo in devices managed by DM Multipath because DM Multipath always resets recovery_tmo when the multipathd service reloads.

5.3. Preventing devices from multipathing

You can configure DM Multipath to ignore selected devices when it configures multipath devices. DM Multipath does not group these ignored devices into a multipath device.

5.3.1. Conditions when DM Multipath creates a multipath device for a path

DM Multipath has a set of default rules to determine whether to create a multipath device for a path or whether to ignore the path. You can configure the behavior.

If the find_multipaths configuration parameter is set to off, multipath always tries to create a multipath device for every path that is not explicitly disabled. If the find_multipaths configuration parameter is set to on, then multipath will create a device only if one of following conditions is met:

  • There are at least two paths with the same WWID that are not disabled.
  • You manually force the creation of the device by specifying a device with the multipath command.
  • A path has the same WWID as a multipath device that was previously created (even if that multipath device does not currently exist). Whenever a multipath device is created, multipath remembers the WWID of the device so that it will automatically create the device again as soon as it sees a path with that WWID. This allows you to have multipath automatically choose the correct paths to make into multipath devices, without have to disable multipathing on other devices.

    If you have previously created a multipath device without using the find_multipaths parameter and then you later set the parameter to on, you may need to remove the WWIDs of any device you do not want created as a multipath device from the /etc/multipath/wwids file. The following shows a sample /etc/multipath/wwids file. The WWIDs are enclosed by slashes (/):

    # Multipath wwids, Version : 1.0
    # NOTE: This file is automatically maintained by multipath and multipathd.
    # You should not need to edit this file in normal circumstances.
    #
    # Valid WWIDs:
    /3600d0230000000000e13955cc3757802/
    /3600d0230000000000e13955cc3757801/
    /3600d0230000000000e13955cc3757800/
    /3600d02300069c9ce09d41c31f29d4c00/
    /SWINSYS  SF2372         0E13955CC3757802/
    /3600d0230000000000e13955cc3757803/

In addition to on and off, you can also set find_multipaths to the following values:

strict
Multipath never accepts paths that have not previously been multipathed and are therefore not in the /etc/multipath/wwids file.
smart
Multipath always accepts non-disabled devices in udev as soon as they appear but if multipathd does not create the device within a timeout set with the find_multipaths_timeout parameter, it will release its claim on the device.

The built-in default value of find_multipaths is off. The default multipath.conf file created by mpathconf, however, will set the value of find_multipaths to on.

With the find_multipaths parameter set to on, you need to disable multipathing only on the devices with multiple paths that you do not want to be multipathed. Because of this, it will generally not be necessary to disable multipathing on devices.

Additional resources

  • For information on the find_multipaths_timeout parameter or the values that you can set for find_multipaths, see the multipath.conf(5) man page.

5.3.2. Criteria for disabling multipathing on certain devices

You can disable multipathing on devices by any of the following criteria:

  • WWID
  • device name
  • device type
  • property
  • protocol

For every device, DM Multipath evaluates these criteria in the following order:

  1. property
  2. devnode
  3. device
  4. protocol
  5. wwid

If a device turns out to be disabled by any criterion, DM Multipath excludes it from handling by multipathd, and does not evaluate the later criteria. For each criterion, the exception list takes precedence over the list of disabled devices if a device matches both.

Additional resources

5.3.3. Disabling multipathing by WWID

You can disable multipathing on individual devices by their World-Wide Identification (WWID).

Procedure

  • Disable devices in the /etc/multipath.conf configuration file using the wwid entry.

    The following example shows the lines in the DM Multipath configuration file that disable a device with a WWID of 26353900f02796769:

    blacklist {
           wwid 26353900f02796769
    }

5.3.4. Disabling multipathing by device name

You can disable multipathing on device types by device name so that DM Multipath will not group them into a multipath device.

Procedure

  • Disable devices in the /etc/multipath.conf configuration file using the devnode entry.

    The following example shows the lines in the DM Multipath configuration file that disable all SCSI devices, because it disables all sd* devices:

    blacklist {
           devnode "^sd[a-z]"
    }

You can use a devnode entry to disable individual devices rather than all devices of a specific type. This is not recommended, however, because unless it is statically mapped by udev rules, there is no guarantee that a specific device will have the same name on reboot. For example, a device name could change from /dev/sda to /dev/sdb on reboot.

By default, DM Multipath disables all devices that are not SCSI, NVMe, or DASD, using the following devnode entry:

blacklist {
       devnode "!^(sd[a-z]|dasd[a-z]|nvme[0-9])"
}

The devices that this entry disables do not generally support DM Multipath.

Additional resources

5.3.5. Disabling multipathing by device type

You can disable multipathing on devices by their device types.

Procedure

  • Disable devices in the /etc/multipath.conf configuration file using the device section.

    The following example disables multipathing on all IBM DS4200 and HP devices:

    blacklist {
           device {
                   vendor  "IBM"
                   product "3S42"       #DS4200 Product 10
           }
           device {
                   vendor  "HP"
                   product ".*"
           }
    }

5.3.6. Disabling multipathing by udev property

You can disable multipathing on devices by their udev environment variable names.

Procedure

  • Disable devices in the /etc/multipath.conf configuration file using the property parameter. The parameter is a regular expression string that matches against the udev environment variable name for the devices.

    The following example disables multipathing on all devices with the udev property ID_ATA:

    blacklist {
            property "ID_ATA"
    }

5.3.7. Disabling multipathing by device protocol

You can disable multipathing on devices by their connection protocols.

Procedure

  1. Optional: View the protocol that a path is using:

    # multipathd show paths format "%d %P"
  2. Disable devices in the /etc/multipath.conf configuration file using the protocol section.

    The following example disables multipathing on all devices with an undefined protocol or an unknown SCSI transport type:

    blacklist {
            protocol "scsi:unspec"
            protocol "undef"
    }

    DM Multipath recognizes the following protocol strings:

    • scsi:fcp
    • scsi:spi
    • scsi:ssa
    • scsi:sbp
    • scsi:srp
    • scsi:iscsi
    • scsi:sas
    • scsi:adt
    • scsi:ata
    • scsi:unspec
    • ccw
    • cciss
    • nvme
    • undef

5.3.8. Adding exceptions for devices with disabled multipathing

You can enable multipathing on devices where multipathing is currently disabled.

Prerequisites

  • Multipathing is disabled on certain devices.

Procedure

  • Enable multipathing on the devices using the blacklist_exceptions section of the /etc/multipath.conf configuration file.

    When specifying devices in the blacklist_exceptions section of the configuration file, you must specify the exceptions using the same criteria as they were specified in the blacklist section. For example, a WWID exception does not apply to devices disabled by a devnode entry, even if the disabled device is associated with that WWID. Similarly, devnode exceptions apply only to devnode entries, and device exceptions apply only to device entries.

    Example 5.1. An exception by WWID

    If you have a large number of devices and want to multipath only one of them (with the WWID of 3600d0230000000000e13955cc3757803), instead of individually disabling each of the devices except the one you want, you could disable all of them, and then enable only the one you want by adding the following lines to the /etc/multipath.conf file:

    blacklist {
            wwid ".*"
    }
    
    blacklist_exceptions {
            wwid "3600d0230000000000e13955cc3757803"
    }

    Alternatively, you could use an exclamation mark (!) to invert the blacklist entry, which disables all devices except the specified WWID:

    blacklist {
            wwid "!3600d0230000000000e13955cc3757803"
    }

    Example 5.2. An exception by udev property

    The property parameter works differently than the other blacklist_exception parameters. If the parameter is set, the device must have a udev variable that matches. Otherwise, the device is disabled. This parameter allows you to disable multipathing on certain SCSI devices, such as USB sticks and local hard drives.

    To enable multipathing only on SCSI devices that could reasonably be multipathed, set this parameter to SCSI_IDENT_|ID_WWN) as in the following example:

    blacklist_exceptions {
            property "(SCSI_IDENT_|ID_WWN)"
    }

5.4. Modifying multipath configuration file defaults

The /etc/multipath.conf configuration file includes a defaults section that sets the user_friendly_names parameter to yes, as follows.

defaults {
        user_friendly_names yes
}

This overwrites the default value of the user_friendly_names parameter.

The configuration file includes a template of configuration defaults. This section is commented out, as follows.

#defaults {
#       polling_interval        10
#       path_selector           "round-robin 0"
#       path_grouping_policy    multibus
#       uid_attribute           ID_SERIAL
#       prio                    alua
#       path_checker            readsector0
#       rr_min_io               100
#       max_fds                 8192
#       rr_weight               priorities
#       failback                immediate
#       no_path_retry           fail
#       user_friendly_names     yes
#}

To overwrite the default value for any of the configuration parameters, you can copy the relevant line from this template into the defaults section and uncomment it. For example, to overwrite the path_grouping_policy parameter so that it is multibus rather than the default value of failover, copy the appropriate line from the template to the initial defaults section of the configuration file, and uncomment it, as follows.

defaults {
        user_friendly_names     yes
        path_grouping_policy    multibus
}

For information on the attributes that are set in the defaults section of the multipath.conf configuration file see the multipath.conf(5) man page. These values are used by DM Multipath unless they are overwritten by the attributes specified in the devices, multipaths, or overrides sections of the multipath.conf file.

5.5. Modifying multipath settings for specific devices

The attributes in the multipaths section of the multipath.conf configuration file apply only to the one specified multipath. These defaults are used by DM Multipath and override attributes set in the overrides, defaults, and devices sections of the multipath.conf file.

For information on the attributes that are set in the multipaths section of the multipath.conf configuration file see the multipath.conf(5) man page.

The following example shows multipath attributes specified in the configuration file for two specific multipath devices. The first device has a WWID of 3600508b4000156d70001200000b0000 and a symbolic name of yellow.

The second multipath device in the example has a WWID of 1DEC_321816758474 and a symbolic name of red. In this example, the rr_weight attributes is set to priorities.

multipaths {
       multipath {
              wwid                  3600508b4000156d70001200000b0000
              alias                 yellow
              path_grouping_policy  multibus
              path_selector         "round-robin 0"
              failback              manual
              rr_weight             priorities
              no_path_retry         5
       }
       multipath {
              wwid                  1DEC_321816758474
              alias                 red
              rr_weight             priorities
        }
}

5.6. Modifying multipath settings for storage controllers

The devices section of the multipath.conf configuration file sets attributes for individual storage devices. These attributes are used by DM Multipath unless they are overwritten by the attributes specified in the multipaths or overrides sections of the multipath.conf file for paths that contain the device. These attributes override the attributes set in the defaults section of the multipath.conf file.

For information on the attributes that are set in the devices section of the multipath.conf configuration file see the multipath.conf(5) man page.

Many devices that support multipathing are included by default in a multipath configuration. For information on the default configuration value, including supported devices, run either of the following commands.

# multipathd show config
# multipath -t

You probably will not need to modify the values for these devices, but if you do you can overwrite the default values by including an entry in the configuration file for the device that overwrites those values. You can copy the device configuration defaults for the device that the multipathd show config command displays and override the values that you want to change.

To add a device that is not configured automatically by default to this section of the configuration file, you need to set the vendor and product parameters. You can find these values by looking at /sys/block/device_name/device/vendor and /sys/block/device_name/device/model where device_name is the device to be multipathed, as in the following example:

# cat /sys/block/sda/device/vendor
WINSYS
# cat /sys/block/sda/device/model
SF2372

The additional parameters to specify depend on your specific device. If the device is active/active, you will usually not need to set additional parameters. You may want to set path_grouping_policy to multibus. Other parameters you may need to set are no_path_retry and rr_min_io.

If the device is active/passive, but it automatically switches paths with I/O to the passive path, you need to change the checker function to one that does not send I/O to the path to test if it is working (otherwise, your device will keep failing over). This almost always means that you set the path_checker to tur; this works for all SCSI devices that support the Test Unit Ready command, which most do.

If the device needs a special command to switch paths, then configuring this device for multipath requires a hardware handler kernel module. The current available hardware handler is emc. If this is not sufficient for your device, you may not be able to configure the device for multipath.

The following example shows a device entry in the multipath configuration file.

#	}
#	device {
#		vendor			"COMPAQ  "
#		product			"MSA1000         "
#		path_grouping_policy	multibus
#		path_checker		tur
#		rr_weight		priorities
#	}
#}

5.7. Setting multipath values for all devices

The overrides section of the multipath.conf configuration file allows you to set a configuration value for all of your devices. For example, you may want all devices to set no_path_retry to fail. This section supports all of the attributes that are supported by both the devices and defaults section of the multipath.conf configuration file, which is all of the devices section attributes except vendor, product and revision. These attributes are used by DM Multipath for all devices unless they are overwritten by the attributes specified in the multipaths section of the multipath.conf file for paths that contain the device. These attributes override the attributes set in the devices and defaults sections of the multipath.conf file.

For information on the attributes that are set in the devices and defaults sections of the multipath.conf configuration file see the multipath.conf(5) man page.

5.8. Additional resources

  • Further information about the DM Multipath configuration file can be found on the multipath.conf(5) man page.

Chapter 6. Managing multipathed volumes

DM-Multipath provides a variety of tools and commands you can use to manage multipath volumes.

6.1. The multipath command

The multipath command is used to detect and coalesce multiple paths to devices. It provides a variety of option you can use to administer your multipathed devices.

Table 6.1, “Useful multipath Command Options” describes some options of the multipath command that you may find useful.

Table 6.1. Useful multipath Command Options

OptionDescription

-l

Display the current multipath configuration gathered from sysfs and the device mapper.

-ll

Display the current multipath configuration gathered from sysfs, the device mapper, and all other available components on the system.

-f device

Remove the named multipath device.

-F

Remove all unused multipath devices.

-w device

Remove the wwid of the specified device from the wwids file.

-W

Reset the wwids file to include only the current multipath devices.

6.1.1. Multipath command output

When you create, modify, or list a multipath device, you get a display of the current device setup. The format is as follows.

For each multipath device:

action_if_any: alias (wwid_if_different_from_alias) dm_device_name_if_known vendor,product size=size features='features' hwhandler='hardware_handler' wp=write_permission_if_known

For each path group:

-+- policy='scheduling_policy' prio=prio_if_known status=path_group_status_if_known

For each path:

 `- host:channel:id:lun devnode major:minor dm_status_if_known path_status online_status

For example, the output of a multipath command might appear as follows:

3600d0230000000000e13955cc3757800 dm-1 WINSYS,SF2372
size=269G features='0' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=1 status=active
| `- 6:0:0:0 sdb 8:16  active ready  running
`-+- policy='round-robin 0' prio=1 status=enabled
  `- 7:0:0:0 sdf 8:80  active ready  running

If the path is up and ready for I/O, the status of the path is ready or ghost. If the path is down, the status is faulty or shaky. The path status is updated periodically by the multipathd daemon based on the polling interval defined in the /etc/multipath.conf file.

Additional possible path status values are as follows.

  • i/o pending: The checker is actively checking this path, and the state will be updated shortly.
  • i/o timeout: This is the same as faulty. It lets the user know that the checker did not return either success or failure before the timeout period.
  • removed: The path has been removed from the system, and will shortly be removed from the multipath device. It is treated the same as faulty.
  • wild: multipathd was unable to run the path checker, because of an internal error or configuration issue. This is roughly the same as faulty, except multipath will skip many actions on the path.
  • unchecked: The path checker has not run on this path, either because it has just been discovered, it does not have an assigned path checker, or the path checker encountered an error. This is treated the same as wild.
  • delayed: The path checker returned that the path is up, but multipath is delaying the reinstatement of the path because the path has recently failed multiple times and multipath has been configured to delay paths in this case.

The dm status is similar to the path status, but from the kernel’s point of view. The active dm state covers the ready and ghost path states. The pending path state has no equivalent dm state. All other path states map to the failed dm state. The dm state will retain its current status until the path checker has completed.

The possible values for online_status are running and offline. A status of offline means that this SCSI device has been disabled.

Note

When a multipath device is being created or modified, the path group status, the dm device name, the write permissions, and the dm status are not known. Also, the features are not always correct.

6.1.2. Displaying multipath configuration

You can use the -l and -ll options of the multipath command to display the current multipath configuration. The -l option displays multipath topology gathered from information in sysfs and the device mapper. The -ll option displays the information the -l option displays in addition to all other available components of the system.

When displaying the multipath configuration, you can specify a verbosity level with the -v option of the multipath command. Specifying -v0 yields no output. Specifying -v1 outputs the created or updated multipath names only, which you can then feed to other tools such as kpartx. Specifying -v2 prints all detected paths, multipaths, and device maps. For even more detailed information, you can also specify -v3, -v4, or -v5.

The following example shows the output of a multipath -l command.

# multipath -l
3600d0230000000000e13955cc3757800 dm-1 WINSYS,SF2372
size=269G features='0' hwhandler='0' wp=rw
|-- policy='round-robin 0' prio=1 status=active
| `- 6:0:0:0 sdb 8:16  active ready  running
`-- policy='round-robin 0' prio=1 status=enabled
  `- 7:0:0:0 sdf 8:80  active ready  running

The following example shows the output of a multipath -ll command.

# multipath -ll
3600d0230000000000e13955cc3757801 dm-10 WINSYS,SF2372
size=269G features='0' hwhandler='0' wp=rw
|-- policy='round-robin 0' prio=1 status=enabled
| `- 19:0:0:1 sdc 8:32  active ready  running
`-- policy='round-robin 0' prio=1 status=enabled
  `- 18:0:0:1 sdh 8:112 active ready  running
3600d0230000000000e13955cc3757803 dm-2 WINSYS,SF2372
size=125G features='0' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=1 status=active
  |- 19:0:0:3 sde 8:64  active ready  running
  `- 18:0:0:3 sdj 8:144 active ready  running

6.2. Resizing an online multipath device

If you need to resize an online multipath device, use the following procedure.

  1. Resize your physical device.
  2. Execute the following command to find the paths to the LUN:

    # multipath -l
  3. Resize your paths. For SCSI devices, writing a 1 to the rescan file for the device causes the SCSI driver to rescan, as in the following command:

    # echo 1 > /sys/block/path_device/device/rescan

    Ensure that you run this command for each of the path devices. For example, if your path devices are sda, sdb, sde, and sdf, you would run the following commands:

    # echo 1 > /sys/block/sda/device/rescan
    # echo 1 > /sys/block/sdb/device/rescan
    # echo 1 > /sys/block/sde/device/rescan
    # echo 1 > /sys/block/sdf/device/rescan
  4. Resize your multipath device by executing the multipathd resize command:

    # multipathd resize map multipath_device
  5. Resize the file system (assuming no LVM or DOS partitions are used):

    # resize2fs /dev/mapper/mpatha

6.3. Moving a root file system from a single path device to a multipath device

If you have installed your system on a single-path device and later add another path to the root file system, you will need to move your root file system to a multipathed device. This section documents the procedure for moving from a single-path to a multipathed device.

After ensuring that you have installed the device-mapper-multipath package, perform the following procedure:

  1. Execute the following command to create the /etc/multipath.conf configuration file, load the multipath module, and set chkconfig for the multipathd to on:

    # mpathconf --enable
  2. If the find_multipaths configuration parameter is not set to yes, edit the blacklist and blacklist_exceptions sections of the /etc/multipath.conf file, as described in Section 5.3, “Preventing devices from multipathing”.
  3. In order for multipath to build a multipath device on top of the root device as soon as it is discovered, enter the following command. This command also ensures that find_multipaths will allow the device, even if it only has one path.

    # multipath -a root_devname

    For example, if the root device is /dev/sdb, enter the following command.

    # multipath -a /dev/sdb
    wwid '3600d02300069c9ce09d41c4ac9c53200' added
  4. To confirm that your configuration file is set up correctly, you can enter the multipath command and search the output for a line of the following format. This indicates that the command failed to create the multipath device.

    date  wwid: ignoring map

    For example, if the WWID if the device is 3600d02300069c9ce09d41c4ac9c53200, you would see a line in the output such as the following:

    # multipath
    Oct 21 09:37:19 | 3600d02300069c9ce09d41c4ac9c53200: ignoring map
  5. To rebuild the initramfs file system with multipath, execute the dracut command with the following options:

    # dracut --force -H --add multipath
  6. Shut the machine down.
  7. Configure the FC switch so that other paths are visible to the machine.
  8. Boot the machine.
  9. Check whether the root file system ('/') is on the multipathed device.

6.4. Moving a swap file system from a single path device to a multipath device

By default, swap devices are set up as logical volumes. This does not require any special procedure for configuring them as multipath devices as long as you set up multipathing on the physical volumes that constitute the logical volume group. If your swap device is not an LVM volume, however, and it is mounted by device name, you may need to edit the /etc/fstab file to switch to the appropriate multipath device name.

  1. Determine the WWID number of the swap device by running the /sbin/multipath command with the -v3 option. The output from the command should show the swap device in the paths list.

    You should look in the command output for a line of the following format, showing the swap device:

    WWID  H:B:T:L devname MAJOR:MINOR

    For example, if your swap file system is set up on sda or one of its partitions, you would see a line in the output such as the following:

    ===== paths list =====
    ...
    1ATA     WDC WD800JD-75MSA3                           WD-WMAM9F 1:0:0:0 sda 8:0
    ...
  2. Set up an alias for the swap device in the /etc/multipath.conf file:

    multipaths {
        multipath {
            wwid WWID_of_swap_device
            alias swapdev
        }
    }
  3. Edit the /etc/fstab file and replace the old device path to the root device with the multipath device.

    For example, if you had the following entry in the /etc/fstab file:

    /dev/sda2 swap                    swap    defaults        0 0

    You would change the entry to the following:

    /dev/mapper/swapdev swap          swap    defaults        0 0

6.5. Determining device mapper entries with the dmsetup command

You can use the dmsetup command to find out which device mapper entries match the multipathed devices.

The following command displays all the device mapper devices and their major and minor numbers. The minor numbers determine the name of the dm device. For example, a minor number of 3 corresponds to the multipathed device /dev/dm-3.

# dmsetup ls
mpathd  (253:4)
mpathep1        (253:12)
mpathfp1        (253:11)
mpathb  (253:3)
mpathgp1        (253:14)
mpathhp1        (253:13)
mpatha  (253:2)
mpathh  (253:9)
mpathg  (253:8)
VolGroup00-LogVol01     (253:1)
mpathf  (253:7)
VolGroup00-LogVol00     (253:0)
mpathe  (253:6)
mpathbp1        (253:10)
mpathd  (253:5)

6.6. Administering the multipathd daemon

The multipathd commands can be used to administer the multipathd daemon. For information on the available multipathd commands, see the multipathd(8) man page.

The following command shows the standard default format for the output of the multipathd show maps command.

# multipathd show maps
name sysfs uuid
mpathc dm-0 360a98000324669436c2b45666c567942

Some multipathd commands include a format option followed by a wildcard. You can display a list of available wildcards with the following command.

# multipathd show wildcards

The multipathd command supports format commands that show the status of multipath devices and paths in "raw" format versions. In raw format, no headers are printed and the fields are not padded to align the columns with the headers. Instead, the fields print exactly as specified in the format string. This output can then be more easily used for scripting. You can display the wildcards used in the format string with the multipathd show wildcards command.

The following multipathd commands show the multipath devices that multipathd is monitoring, using a format string with multipath wildcards, in regular and raw format.

list|show maps|multipaths format $format
list|show maps|multipaths raw format $format

The following multipathd commands show the paths that multipathd is monitoring, using a format string with multipath wildcards, in regular and raw format.

list|show paths format $format
list|show paths raw format $format

The following commands show the difference between the non-raw and raw formats for the multipathd show maps. Note that in raw format there are no headers and only a single space between the columns.

# multipathd show maps format "%n %w %d %s"
name   uuid                              sysfs vend/prod/rev
mpathc 360a98000324669436c2b45666c567942 dm-0  NETAPP,LUN

# multipathd show maps raw format "%n %w %d %s"
mpathc 360a98000324669436c2b45666c567942 dm-0 NETAPP,LUN

6.7. Cleaning up multipath files on package removal

If you should have occasion to remove the device-mapper-multipath rpm. file, note that this does not remove the /etc/multipath.conf, /etc/multipath/bindings, and /etc/multipath/wwids files. You may need to remove those files manually on subsequent installations of the device-mapper-multipath package.

Chapter 7. Removing storage devices

You can safely remove a storage device from a running system, which helps prevent system memory overload and data loss.

Note

Before you remove a storage device, you must ensure that you have enough free system memory due to the increased system memory load during an I/O flush. Use the vmstat 1 100 command to view the system’s current memory load. You can also use the free command to view the system’s free memory. Red Hat does not recommend removing a storage device on a system where:

  • Free memory is less than 5% of the total memory in more than 10 samples per 100.
  • Swapping is active (non-zero si and so columns in the vmstat command output).

7.1. Safe removal of storage devices

Safely removing a storage device from a running system requires a top-to-bottom approach, starting from the top layer, which typically is an application or a filesystem, and working towards the bottom layer, which is the physical device.

You can use storage devices in multiple ways, and they can have different virtual configurations on top of physical devices. For example, you can configure a virtual device as a multipath device, make it part of a RAID, or you can make it part of an LVM group. Alternatively, devices can be accessed via a filesystem, or they can be accessed directly (as a “raw” device).

Working from the top layer down, you must ensure that the device that you want to remove is not in use, all pending I/O to the device is flushed, and the operating system is not referencing the storage device.

7.2. Removing a block device

You can safely remove a block device from a running system to help prevent system memory overload and data loss.

Warning

Rescanning the SCSI bus or performing any other action that changes the state of the operating system’s state without following the procedure documented here can cause delays due to I/O timeouts, devices to be removed unexpectedly, or data loss.

Prerequisites

  • If you want to remove a multipath device, and you are unable to access its path devices, disable the multipath device’s queue:

    # multipathd disablequeueing map <multipath-device>

    This enables the device’s I/O to fail, allowing the applications that are using the device to shut down.

Procedure

  1. Ensure that no other applications or services are using the device that you want to remove.
  2. Back up the data from the device that you want to remove.
  3. Unmount any filesystems that are mounted on the device using the umount command.
  4. Remove the device from any md RAID array or from any LVM volume that it belongs to. For example, if the device is a member of an LVM group, and it is a multipath device:

    1. Move the data to another device:

      # pvmove -b /dev/mapper/<from-multipath-device> /dev/mapper/<to-multipath-device>
    2. Remove the device from the volume group:

      # vgreduce <volume-group> /dev/mapper/<from-multipath-device>
    3. (Optional) Remove the LVM metadata from the physical device:

      # pvremove /dev/mapper/<from-multipath-device>
  5. If you are removing a multipath device:

    1. View all the paths to the device using the following command:

      # multipath -l

      The output of this command is required in a later step.

    2. Flush the I/O and remove the multipath device:

      # multipath -f <multipath-device>
  6. If the device is not configured as a multipath device, or if the device is configured as a multipath device and you have previously passed I/O to its individual paths, flush any outstanding I/O to all device paths that are used:

    # blockdev --flushbufs <device>

    This is important for devices accessed directly where the umount or vgreduce commands do not flush the I/O.

  7. If you are removing a SCSI device, complete the following steps:

    1. Remove any reference to the device’s path-based name; for example, /dev/sd, /dev/disk/by-path or the major:minor number, in applications, scripts, or utilities on the system. This ensures that different devices added in the future are not mistaken for the current device.
    2. Remove each path to the device from the SCSI subsystem:

      # echo 1 > /sys/block/<device-name>/device/delete

      where <device-name> is retrieved from the output of the multipath -l command in step 5 if the device was previously used as a multipath device.

  8. Remove the physical device form a running system. Note that the I/O to other devices does not stop when you remove this device.

Additional resources

  • The multipath, pvmove, vgreduce, blockdev, and umount man pages.

Chapter 8. Troubleshooting DM Multipath

If you have trouble implementing a multipath configuration, there are a variety of issues you can check for.

8.1. DM Multipath troubleshooting checklist

The following issues may result in a slow or non-functioning multipath configuration.

8.1.1. The multipath daemon is not running

If you find you have trouble implementing a multipath configuration, you should ensure that the multipath daemon is running, as described in Chapter 3, Setting up DM Multipath.

The multipathd daemon must be running in order to use multipathed devices.

8.1.2. Issues with queue_if_no_path feature

If a multipath device is configured with features "1 queue_if_no_path", then any process that issues I/O will hang until one or more paths are restored. To avoid this, set the no_path_retry N parameter in the /etc/multipath.conf file (where N is the number of times the system should retry a path).

If you need to use the features "1 queue_if_no_path" option and you experience the issue noted here, you can disable queueing policy at runtime for a particular LUN (that is, for which all the paths are unavailable). The following command disables queueing for a specific device.

multipathd disablequeueing map device

The following command disables queueing for all devices.

multipathd disablequeueing maps

After you have disabled queueing for a device, it will remain disabled until multipathd is restarted or reloaded or until you one of the following commands.

The following command resets queueing to its previous value for a specific device.

multipathd restorequeueing map device

The following command resets queueing to its previous value for all devices.

multipathd restorequeueing maps

8.2. Troubleshooting with the multipathd interactive console

The multipathd -k command is an interactive interface to the multipathd daemon. Entering this command brings up an interactive multipath console. After executing this command, you can enter help to get a list of available commands, you can enter a interactive command, or you can enter CTRL-D to quit.

The multipathd interactive console can be used to troubleshoot problems you may be having with your system. For example, the following command sequence displays the multipath configuration, including the defaults, before exiting the console.

# multipathd -k
> > show config
> > CTRL-D

The following command sequence ensures that multipath has picked up any changes to the multipath.conf,

# multipathd -k
> > reconfigure
> > CTRL-D

Use the following command sequence to ensure that the path checker is working properly.

# multipathd -k
> > show paths
> > CTRL-D

Chapter 9. Configuring maximum time for storage error recovery with eh_deadline

You can configure the maximum allowed time to recover failed SCSI devices. This configuration guarantees an I/O response time even when storage hardware becomes unresponsive due to a failure.

9.1. The eh_deadline parameter

The SCSI error handling (EH) mechanism attempts to perform error recovery on failed SCSI devices. The SCSI host object eh_deadline parameter enables you to configure the maximum amount of time for the recovery. After the configured time expires, SCSI EH stops and resets the entire host bus adapter (HBA).

Using eh_deadline can reduce the time:

  • to shut off a failed path,
  • to switch a path, or
  • to disable a RAID slice.
Warning

When eh_deadline expires, SCSI EH resets the HBA, which affects all target paths on that HBA, not only the failing one. If some of the redundant paths are not available for other reasons, I/O errors might occur. Enable eh_deadline only if you have a fully redundant multipath configuration on all targets.

Scenarios when eh_deadline is useful

In most scenarios, you do not need to enable eh_deadline. Using eh_deadline can be useful in certain specific scenarios, for example if a link loss occurs between a Fibre Channel (FC) switch and a target port, and the HBA does not receive Registered State Change Notifications (RSCNs). In such a case, I/O requests and error recovery commands all time out rather than encounter an error. Setting eh_deadline in this environment puts an upper limit on the recovery time. That enables the failed I/O to be retried on another available path by DM Multipath.

Under the following conditions, the eh_deadline functionality provides no additional benefit, because the I/O and error recovery commands fail immediately, which allows DM Multipath to retry:

  • If RSCNs are enabled
  • If the HBA does not register the link becoming unavailable

Possible values

The value of the eh_deadline is specified in seconds.

The default setting is off, which disables the time limit and allows all of the error recovery to take place.

9.2. Setting the eh_deadline parameter

This procedure configures the value of the eh_deadline parameter to limit the maximum SCSI recovery time.

Procedure

  • You can configure eh_deadline using either of the following methods:

    sysfs
    Write the number of seconds into the /sys/class/scsi_host/host*/eh_deadline files.
    Kernel parameter
    Set a default value for all SCSI HBAs using the scsi_mod.eh_deadline kernel parameter.

Legal Notice

Copyright © 2021 Red Hat, Inc.
The text of and illustrations in this document are licensed by Red Hat under a Creative Commons Attribution–Share Alike 3.0 Unported license ("CC-BY-SA"). An explanation of CC-BY-SA is available at http://creativecommons.org/licenses/by-sa/3.0/. In accordance with CC-BY-SA, if you distribute this document or an adaptation of it, you must provide the URL for the original version.
Red Hat, as the licensor of this document, waives the right to enforce, and agrees not to assert, Section 4d of CC-BY-SA to the fullest extent permitted by applicable law.
Red Hat, Red Hat Enterprise Linux, the Shadowman logo, the Red Hat logo, JBoss, OpenShift, Fedora, the Infinity logo, and RHCE are trademarks of Red Hat, Inc., registered in the United States and other countries.
Linux® is the registered trademark of Linus Torvalds in the United States and other countries.
Java® is a registered trademark of Oracle and/or its affiliates.
XFS® is a trademark of Silicon Graphics International Corp. or its subsidiaries in the United States and/or other countries.
MySQL® is a registered trademark of MySQL AB in the United States, the European Union and other countries.
Node.js® is an official trademark of Joyent. Red Hat is not formally related to or endorsed by the official Joyent Node.js open source or commercial project.
The OpenStack® Word Mark and OpenStack logo are either registered trademarks/service marks or trademarks/service marks of the OpenStack Foundation, in the United States and other countries and are used with the OpenStack Foundation's permission. We are not affiliated with, endorsed or sponsored by the OpenStack Foundation, or the OpenStack community.
All other trademarks are the property of their respective owners.