Chapter 25. Discarding unused blocks

• the system’s workload is such that batch discard is not feasible, or
• online discard operations are necessary to maintain performance.

Prerequisites

• The file system is mounted.
• The block device underlying the file system supports physical discard operations.

Procedure

• Use the fstrim utility:

  • To perform discard only on a selected file system, use:

    # fstrim mount-point

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  • To perform discard on all mounted file systems, use:

    # fstrim --all

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• If you run the fstrim command on:

  • a device that does not support discard operations, or

  • a logical device (LVM or MD) composed of multiple devices, where any one of the device does not support discard operations, the following message displays:

    # fstrim /mnt/non_discard
    
    fstrim: /mnt/non_discard: the discard operation is not supported

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Procedure

• Enable online discard at mount time:

  • When mounting a file system manually, add the -o discard mount option:

    # mount -o discard device mount-point

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  • When mounting a file system persistently, add the discard option to the mount entry in the /etc/fstab file.

Procedure

• Enable and start the systemd timer:

  # systemctl enable --now fstrim.timer
  Created symlink /etc/systemd/system/timers.target.wants/fstrim.timer  /usr/lib/systemd/system/fstrim.timer.

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Verification

• Verify the status of the timer:

  # systemctl status fstrim.timer
  fstrim.timer - Discard unused blocks once a week
     Loaded: loaded (/usr/lib/systemd/system/fstrim.timer; enabled; vendor preset: disabled)
     Active: active (waiting) since Wed 2023-05-17 13:24:41 CEST; 3min 15s ago
    Trigger: Mon 2023-05-22 01:20:46 CEST; 4 days left
       Docs: man:fstrim
  
  May 17 13:24:41 localhost.localdomain systemd[1]: Started Discard unused blocks once a week.

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