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