Set the disk scheduler using udev rules

You can set a given disk scheduler for specific block devices by using the udev rules. The setting persists across system reboots.

About this task

When configuring udev rules to set a disk scheduler, replace:

  • device with the name of the block device, for example sdf and
  • selected-scheduler with the disk scheduler that you want to set for the device, for example bfq.

Procedure

  1. Find the system unique identifier of the block device: 
    $ udevadm info --name=/dev/device | grep -E '(WWN|SERIAL)'
     
    E: ID_WWN=0x5002538d00000000
E: ID_SERIAL=Generic-_SD_MMC_20120501030900000-0:0
E: ID_SERIAL_SHORT=20120501030900000
    Note
    This command returns all values identified as a World Wide Name (WWN) or serial number associated with the specified block device. Although using a WWN is preferred, it is not always available for every device. In such cases, you can use any value returned by the example command as the device system’s unique ID.
  2. To configure the udev rule, create the /etc/udev/rules.d/99-scheduler.rules file with the following content: 
    ACTION=="add|change", SUBSYSTEM=="block", ENV{IDNAME}=="device system unique id", ATTR{queue/scheduler}="selected-scheduler"

    Here:

    • Replace IDNAME with the name of the identifier being used (for example, ID_WWN).
    • Replace device system unique id with the value of the chosen identifier (for example, 0x5002538d00000000).
  3. Reload udev rules: 
    # udevadm control --reload-rules
  4. Apply the scheduler configuration: 
    # udevadm trigger --type=devices --action=change

Results

  • Verify the active scheduler: 
    # cat /sys/block/device/queue/scheduler