The amount of space consumed by this dataset and all its descendents. This value is checked against the dataset's quota and reservation. The space used does not include the dataset's reservation, but does consider the reservation of any descendent datasets. The amount of space that a dataset consumes from its parent, as well as the amount of space that is freed if the dataset is recursively destroyed, is the greater of its space used and its reservation.
When snapshots are created, their space is initially shared between the snapshot and the file system, and possibly with previous snapshots. As the file system changes, space that was previously shared becomes unique to the snapshot, and counted in the snapshot's space used. The space that is used by a snapshot accounts for its unique data. Additionally, deleting snapshots can increase the amount of space unique to (and used by) other snapshots. For more information about snapshots and space issues, see Out of Space Behavior.
The amount of space used, available, or referenced does not take into account pending changes. Pending changes are generally accounted for within a few seconds. Committing a change to a disk using fsync(3c) or O_SYNC does not necessarily guarantee that the space usage information will be updated immediately.
The usedbychildren, usedbydataset, usedbyrefreservation, and usedbysnapshots property information can be displayed with the zfs list -o space command. These properties break down the used property into space that is consumed by descendents. For more information, see Table 6–1.