Snapshots and Data Consistency

The ZFSSA replicates snapshots and each snapshot is received atomically on the target, so the contents of a share's replica on the target always matches the share's contents on the source at the time the snapshot was taken. Because the snapshots for all shares sent in a particular group are taken at the same time (see above), the entire package contents after the completion of a successful replication update exactly matches the group's content when the snapshot was created on the source (when the replication update began).

However, each share's snapshots are replicated separately (and serially), so it's possible for some shares within a package to have been updated with a snapshot that is more recent than those of other shares in the same package. This is true during a replication update (after some shares have been updated but before others have) and after a failed replication update (after which some shares may have been updated but others may not have been).

To summarize:

• Each share is always point-in-time consistent on the target (self-consistent).

• When no replication update is in progress and the previous replication update succeeded, each package's shares are also point-in-time consistent with each other (package-consistent).

• When a replication update is in progress or the previous update failed, package shares may be inconsistent with each other, but each one will still be self-consistent. If package consistency is important for an application, one must clone the replication package, which always clones the most recent successfully received snapshot of each share.