Sam Mason wrote:
My original note was mainly in response to Craig's comment that implied fsync doing far more than it actually does. I remember seeing a few comments recently saying similar things about fsync, so sorry for picking specifically on you Craig. Device/filesystem level snapshotting is exactly what's needed and is independent of any fsync settings.
Good point. I guess fsync would only matter for certain if you were using snapshots on shared storage (a SAN or similar) where the current host's view of the yet-to-be-flushed-to-storage FS state is not known.
The main snapshot system I use is LVM, which takes a snapshot of the block device underlying the file system. It's not actually filesystem level at all, but volume/block level. I'm just not sure whether filesystems like ext3 and the underlying Linux VFS will always pass blocks written by applications through to the block layer immediately and in order. After all, they do have tricks like delayed allocation that I'd expect to result in data not being passed to the file system. If they don't always pass writes to the block layer in the order they're issued then fsync would also be significant for LVM snapshots. Any idea?
ext3's data=ordered refers to data being written before metadata, not the order in which different data write requests are processed, so that's no help.
-- Craig Ringer
