2.4. Discard unused blocks

Batch discard and online discard operations are features of mounted file systems that discard blocks not in use by the file system. They are useful for both solid-state drives and thinly-provisioned storage.

Batch discard operations are run explicitly by the user with the fstrim command. This command discards all unused blocks in a file system that match the user's criteria. Both operation types are supported for use with ext4 file systems as of Red Hat Enterprise Linux 6.2 and later, so long as the block device underlying the file system supports physical discard operations. This is also the case with XFS file systems as of Red Hat Enterprise Linux 6.4 and later. Physical discard operations are supported if the value of /sys/block/device/queue/discard_max_bytes is not zero.

Online discard operations are specified at mount time with the -o discard option (either in /etc/fstab or as part of the mount command), and run in realtime without user intervention. Online discard operations only discard blocks that are transitioning from used to free. Online discard operations are supported on ext4 file systems as of Red Hat Enterprise Linux 6.2 and later, and on XFS file systems as of Red Hat Enterprise Linux 6.4 and later.

Red Hat recommends batch discard operations unless the system's workload is such that batch discard is not feasible, or online discard operations are necessary to maintain performance.