On 7/16/10 12:02 PM -0500 David Dyer-Bennet wrote:
It would be nice to have applications request to be notified
before a snapshot is taken, and when that have requested
notification have acknowledged that they're ready, the snapshot
would be taken; and then another notification sent that it was
taken. Prior to indicating they were ready, the apps could
have achieved a logically consistent on disk state. That
would eliminate the need for (for example) separate database
backups, if you could have a snapshot with the database on it
in a consistent state.
Any software dependent on cooperating with the filesystem to ensure that
the files are consistent in a snapshot fails the cord-yank test (which is
equivalent to the "processor explodes" test and the "power supply bursts
into flames" test and the "disk drive shatters" test and so forth). It
can't survive unavoidable physical-world events.
It can, if said software can roll back to the last consistent state.
That may or may not be "recent" wrt a snapshot. If an application is
very active, it's possible that many snapshots may be taken, none of
which are actually in a state the application can use to recover from.
Rendering snapshots much less effective.
Also, just administratively, and perhaps legally, it's highly desirable
to know that the time of a snapshot is the actual time that application
state can be recovered to or referenced to.
Also, if an application cannot survive a cord-yank test, it might be
even more highly desirable that snapshots be a stable that from which
the application can be restarted.
A notification mechanism is pretty desirable, IMHO.
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
