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Chapter 22. Discarding unused blocks

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You can perform or schedule discard operations on block devices that support them. The block discard operation communicates to the underlying storage which filesystem blocks are no longer in use by the mounted filesystem. Block discard operations allow SSDs to optimize garbage collection routines, and they can inform thinly-provisioned storage to repurpose unused physical blocks.

Requirements

  • The block device underlying the file system must support physical discard operations.

    Physical discard operations are supported if the value in the /sys/block/<device>/queue/discard_max_bytes file is not zero.

22.1. Types of block discard operations

You can run discard operations using different methods:

Batch discard
Is triggered explicitly by the user and discards all unused blocks in the selected file systems.
Online discard
Is specified at mount time and triggers in real time without user intervention. Online discard operations discard only blocks that are transitioning from the used to the free state.
Periodic discard
Are batch operations that are run regularly by a systemd service.

All types are supported by the XFS and ext4 file systems.

Recommendations

Red Hat recommends that you use batch or periodic discard.

Use online discard only if:

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

22.2. Performing batch block discard

You can perform a batch block discard operation to discard unused blocks on a mounted file system.

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

      # fstrim --all

If you execute 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

Additional resources

  • fstrim(8) man page.

22.3. Enabling online block discard

You can perform online block discard operations to automatically discard unused blocks on all supported file systems.

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

Additional resources

  • mount(8) man page.
  • fstab(5) man page.

22.4. Enabling periodic block discard

You can enable a systemd timer to regularly discard unused blocks on all supported file systems.

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.

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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