Chapter 9. Freeing space on thinly-provisioned logical volumes using fstrim

You can manually run fstrim to return unused logical volume space to the thin pool, which can be available for other logical volumes.

Red Hat recommends running fstrim daily.

Prerequisites

  • Verify that the thin pool logical volume supports discard behavior.

    Discard is supported if the output of the following command for the underlying device is not zero. 

    # cat /sys/block/<device>/queue/discard_max_bytes
    Note

    If discard_max_bytes returns 0, you can still execute the fstrim command on the thin logical volume to reclaim unused blocks to the thin pool. However, the discard or trim will not be passed down to the block layer.

Procedure

  1. Run fstrim to restore physical space to the thin pool. 

    # fstrim -v <mountpoint>

    For example, the following command discards any unused space it finds on the logical volume mounted at /gluster_bricks/data/data, and provides verbose output (-v). 

    # fstrim -v /gluster_bricks/data/data