New hammer utility feature in HEAD: 'hammer cleanup'

[Matthew Dillon]

:When run on a filesystem without snapshots, cleanup seems to create a
:single snapshot and start pruning.
:>From the man page I would have expected a snapshot per day (also for
:previous days).
:I think with the current setup one can loose history if cleanup is
:not run often enough.
:
:Regards,
:Johannes

    Yes, I see what you are saying.  Since the idea is to run cleanup
    from cron the issue only really applies to the initial cleanup on
    a filesystem that has never been cleaned before.  I think the
    current behavior is reasonable if a bit unexpected, but it is really
    only a one-time thing.

    It is possible to generate backdated snapshots for a filesystem that
    has never been pruned, or to generate backdated snapshots from the
    most recent snapshot onward (if, say, you had stopped cleaning for
    several days).

    HAMMER doesn't record the highest transaction id pruned though and
    creating snapshots for older transaction ids (in areas already pruned)
    will not give you a consistent snapshot so 'catch-up' snapshots might
    not reflect consistent views of the filesystem.  In otherwords,
    the catch-up snapshots might not contain the expected results.

    --

    I don't plan on doing this but if someone has a coding itch I can tell
    you how to implement it:

    * Initiate a mirror-read from the starting transaction id.  That is,
      the transaction id of the most recent snapshot, or (1) if no snapshots
      are present.  NOTE: it is not possible to obtain consistent snapshots
      for any period prior to the point where a filesystem has been pruned
      via 'prune-everything'.

    * Collect transaction ids and datestamps (the B-Tree elements have
      both).  Create a histogram by using the datestamps to sort the
      transaction ids into hoppers, using a 10 minute granularity.

    * Select transaction ids in the rough time periods you wish snapshotted,
      try to look for a period of quiescence so the snapshot is consistent
      (with the above caveat: a consistent snapshot is not possible for
      any period of time covered by a prior pruning operation).

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>