On Fri, July 16, 2010 14:07, Frank Cusack wrote: > 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.
Wait, if the application can in fact survive the "cord pull" test then by definition of "survive", all the snapshots are useful. They'll be everything consistent that was committed to disk by the time of the yank (or snapshot); which, it seems to me, is the very best that anybody could hope for. > 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. Maybe, but since that's not achievable for your core corporate asset (the database), I think of it as a pipe dream rather than a goal. > 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. If it cannot survive a cord-yank test, it should not be run, ever, by anybody, for any purpose more important than playing a game. -- David Dyer-Bennet, d...@dd-b.net; http://dd-b.net/ Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/ Photos: http://dd-b.net/photography/gallery/ Dragaera: http://dragaera.info _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
