When a job is started, the snapshotter attempts to get the snapshotting rhythms of the matched ``filesystems`` in sync because snapshotting all filesystems at the same time results in a more consistent backup.
To find that sync point, the most recent snapshot, made by the snapshotter, in any of the matched ``filesystems`` is used.
A filesystem that does not have snapshots by the snapshotter has lower priority than filesystem that do, and thus might not be snapshotted (and replicated) until it is snapshotted at the next sync point.
Jobs with `periodic snapshots <job-snapshotting-spec_>`_ can run hooks before and/or after taking the snapshot specified in ``snapshotting.hooks``:
Hooks are called per filesystem before and after the snapshot is taken (pre- and post-edge).
Pre-edge invocations are in configuration order, post-edge invocations in reverse order, i.e. like a stack.
If a pre-snapshot invocation fails, ``err_is_fatal=true`` cuts off subsequent hooks, does not take a snapshot, and only invokes post-edges corresponding to previous successful pre-edges.
``err_is_fatal=false`` logs the failed pre-edge invocation but does not affect subsequent hooks nor snapshotting itself.
Post-edges are only invoked for hooks whose pre-edges ran without error.
Note that hook failures for one filesystem never affect other filesystems.
The optional ``timeout`` parameter specifies a period after which zrepl will kill the hook process and report an error.
The default is 30 seconds and may be specified in any units understood by `time.ParseDuration <https://golang.org/pkg/time/#ParseDuration>`_.
The optional ``filesystems`` filter which limits the filesystems the hook runs for. This uses the same |filter-spec| as jobs.
Most hook types take additional parameters, please refer to the respective subsections below.
Connects to a Postgres server and executes the ``CHECKPOINT`` statement pre-snapshot.
Checkpointing applies the WAL contents to all data files and syncs the data files to disk.
This is not required for a consistent database backup: it merely forward-pays the "cost" of WAL replay to the time of snapshotting instead of at restore.
However, the Postgres manual recommends against checkpointing during normal operation.
Further, the operation requires Postgres superuser privileges.
zrepl users must decide on their own whether this hook is useful for them (it likely isn't).
..ATTENTION::
Note that WALs and Postgres data directory (with all database data files) must be on the same filesystem to guarantee a correct point-in-time backup with the ZFS snapshot.
* pre-snapshot ``FLUSH TABLES WITH READ LOCK`` to lock all tables in all databases in the MySQL server we connect to (`docs <https://dev.mysql.com/doc/refman/8.0/en/flush.html#flush-tables-with-read-lock>`_)
