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Chapter 19. Discarding unused blocks

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

19.1. Types of block discard operations

You can run discard operations using different methods:

Batch discard
This type of discard is part of the fstrim command. It discards all unused blocks in a file system that match criteria specified by the administrator. Red Hat Enterprise Linux 10 supports batch discard on XFS and ext4 formatted devices that support physical discard operations.
Online discard

This type of discard operation is configured at mount time with the discard option, and runs in real time without user intervention. However, it only discards blocks that are transitioning from used to free. Red Hat Enterprise Linux 10 supports online discard on XFS and ext4 formatted devices.

Use batch discard, except when online discard is required to maintain performance, or when batch discard is not feasible for the workload of the system.

Periodic discard
Batch operations that are run regularly by a systemd service.

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

Recommendations

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.

19.2. 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
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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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19.3. 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
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    • When mounting a file system persistently, add the discard option to the mount entry in the /etc/fstab file.

19.4. Enabling online block discard by using the storage RHEL system role

You can mount an XFS file system with the online block discard option to automatically discard unused blocks.

Prerequisites

Procedure

  1. Create a playbook file, for example ~/playbook.yml, with the following content: 

    ---
- name: Manage local storage
  hosts: managed-node-01.example.com
  tasks:
    - name: Enable online block discard
      ansible.builtin.include_role:
        name: redhat.rhel_system_roles.storage
      vars:
        storage_volumes:
          - name: barefs
            type: disk
            disks:
              - /dev/sdb
            fs_type: xfs
            mount_point: /mnt/data
            mount_options: discard
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    For details about all variables used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.storage/README.md file on the control node.

  2. Validate the playbook syntax: 

    $ ansible-playbook --syntax-check ~/playbook.yml
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    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook: 

    $ ansible-playbook ~/playbook.yml
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Verification

  • Verify that online block discard option is enabled: 

    # ansible managed-node-01.example.com -m command -a 'findmnt /mnt/data'
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19.5. 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.
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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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