Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
On Thu, 14 Feb 2008, H.D. wrote: >> I disagree. It's not a solution because syncing the FS isn't sufficient. >> With LVM you still get the FS level consistency as it is at the time of the >> snapshot. The problem is that applications like DBs maintain their internal >> buffers and caches, and these will not necessarily be consistent with the >> data on disk, FS freeze or not. > > Syncing the FS _is_ sufficient. That is why DBMS implement write ahead logs. > Recovering from a snapshot is the same as recovering from a power failure. > And if a DBMS can't do that, it doesn't conform to ACID, and no one serious > would use it. There is no serious DBMS out there that is unhappy with FS > snapshots, even MySQL / innoDB is fine. Syncing the FS is sufficient for data safety on the level of journalling that keeps the FS consistent after a power failure. It is NOT sufficient for data consistency I would expect from a proper backup. > In case of XFS it's actually better than a power failure, both meta data and > data is flushed, as well as an umount record written. Many file systems have data AND metadata journalling available, including ext3 (even if ext3 by default only journals metadata). > Have you ever tried running a DBMS on copyfs? There is a reason why no one is > doing that. First there is no reason to do it, second performance is > unacceptable. No, it never occured to me to run a DBMS on CopyFS. As you quite rightly point out, there is no point in doing it. As for DB backups, backup up a replicated server is a much safer and more sensible option than snapshotting. Even if snapshotting provided consistent images (which it doesn't), it still impairs the performance of a live server (it effectively does the same thing CopyFS does). Having a separate replicated DB server gives you both the guaranteed consistency and doesn't have a serious performance hit on the master server. Hmm... I think we've drifted a tad far off the DRBD topic... Gordan