New hammer utility feature in HEAD: 'hammer cleanup'
dillon at apollo.backplane.com
Sat Sep 27 23:53:29 PDT 2008
: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
:I think with the current setup one can loose history if cleanup is
:not run often enough.
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
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
* 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).
<dillon at backplane.com>
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