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Chapter 8. Removing storage devices

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

Prerequisites

  • 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 following commands to view the current memory load and free memory of the system: 

    # vmstat 1 100
# free

  • 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).

8.1. Safe removal of storage devices

Safely removing a storage device from a running system requires a top-to-bottom approach. Start from the top layer, which typically is an application or a file system, and work 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 group multiple instances of a device into a multipath device, make it part of a RAID, or you can make it part of an LVM group. Additionally, devices can be accessed via a file system, or they can be accessed directly such as a “raw” device.

While using the top-to-bottom approach, you must ensure that:

  • the device that you want to remove is not in use
  • all pending I/O to the device is flushed
  • the operating system is not referencing the storage device

8.2. 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, 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 queueing of the multipath device: 

    # multipathd disablequeueing map multipath-device

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

  • Ensure that no other applications or services are using the device that you want to remove.
  • Ensure that you back up the data from the device that you want to remove.

Procedure

  1. Unmount any file systems that are mounted on the device using the umount command.

  2. Remove the device from any MD RAID array or from any LVM volume that it belongs to. Depending on the device type, execute one of the following steps:

    • 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

    • If you are removing a multipath device, execute the following commands:

      1. View all the paths to the device: 

        # 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

    • 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 the 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.

    • If you are removing a SCSI device, execute the following commands:

      1. Remove any reference to the path-based name of the device, such as /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, if the device was previously used as a multipath device.

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

Additional resources

  • The multipath(8), pvmove(8), vgreduce(8), blockdev(8) and umount(8) man pages.