Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
OK, thank you for your detailed answer again, it really helps me! On 15.02.2007 14:34, Graham Wood wrote: > >> To me that reads `LVM is only capable of creating an "almost >> consistent" snapshot'. > Of a running MySQL database... The issue is at the MySQL level, rather > than the LVM level. Without grabbing the read lock that document > mentions, you can easily have transactions partially commited to disk, > without a consistent state on disk. > >> Is that true? > Restoring from a snapshot is the same (almost) as recovering from a > power failure at the time the snapshot was taken. If (for example) > MySQL is in the process of commiting a transaction in 2 places on the > disk when the snapshot is taken, because it is instantaneous you might > only have 1 of them commited at the time the snapshot is taken, and > it'll be inconsistent. > > In the case of Oracle you can put the database into a hot backup mode, > take the snapshot, and then take it out. When you do the recovery you > can then replay the log it generated in that time to recover. This was > intended for tape backups (since they can take a long time, and > therefore will NOT be consistent), and actually allows you to replay to > any time between start and finish.... > > Looking on google for references to postgreSQL and snapshot backups, it > looks like you've got the same functionality as provided by InnoDB on > MySQL (and pretty much every other DB out there) - log replay > functionality all the time. > > As long as the logs are snapshotted at the same time as the main > database, on recovery it can replay the log to get back to a consistent > state. If you've got the log files on a different device to the main > database then you ideally want to snapshot the two at exactly the same > time (theoretically impossible).... > > However, I think if you snapshot the main device, and then the log > files, you are almost definitely going to be "OK". The only problem > would be if the logs had rotated and lost information from when the main > devices were snapshotted. At that point, your snapshots are going to be > smoking anyway, so you're going to have other problems. > > Graham -- Regards, H.D.