USED for ZFS clone filesystems

[Bournenn]

What is USED for ZFS clone filesystems.Why are clone filesystems created for the same snapshot at different times (during which the filesystem data is read and written, and the snapshot size changes) different?The following figure shows a cloned filesystem created for the same snapshot:

Thanks!

[amotin]

man zfsprops:

     used                  The amount of space consumed by this dataset and
                           all its descendents.  This is the value that is
                           checked against this dataset's quota and
                           reservation.  The space used does not include this
                           dataset's reservation, but does take into account
                           the reservations 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 this dataset is recursively destroyed, is
                           the greater of its space used and its reservation.

                           The used space of a snapshot (see the Snapshots
                           section of zfsconcepts(7)) is space that is
                           referenced exclusively by this snapshot.  If this
                           snapshot is destroyed, the amount of used space
                           will be freed.  Space that is shared by multiple
                           snapshots isn't accounted for in this metric.  When
                           a snapshot is destroyed, space that was previously
                           shared with this snapshot can become unique to
                           snapshots adjacent to it, thus changing the used
                           space of those snapshots.  The used space of the
                           latest snapshot can also be affected by changes in
                           the file system.  Note that the used space of a
                           snapshot is a subset of the written space of the
                           snapshot.

                           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(2) or O_SYNC does not necessarily guarantee
                           that the space usage information is updated
                           immediately.

[Bournenn]

Thanks for your answer.But I still don't understand why the clone filesystems created by the same snapshot are not the same size. I haven't done any write or modification to these clone filesystems.

[Bournenn]

Another problem is that the clone filesystem starts at 0 and grows as the snapshot size increases.

[rincebrain]

You'd have to really drill in to look, but I would assume it's that the tiny amount of metadata that's generated to make a clone compresses variably well.

Why would it start at something other than near-zero USED? The point of a clone is that it's not an independent copy of the "origin", so it shouldn't take up separate disk space except as things change.