This is something I've never quite understood. Best practice guides say you need to do a "flush all tables" in MySQL and then do a filesystem freeze (possible in XFS) before you can use a snapshot system like the ones built into EBS or LVM. If you don't, you apparently stand a good chance of getting an inconsistent snapshot, even if the snapshotting mechanism itself is (like EBS and LVM) "point in time" consistent.
Why is all this necessary? If the system (i.e. DB + FS + block device) are all working as they should, then once a commit returns, the data should be on disk. If it's not, you have no guarantee data that you thought was committed will still be there after a kernel panic or power outage.
In that case, no amount of xfs-freeze or table flushing during a snapshot is going to save you from the fact that your DB is one kernel panic away from losing what the rest of your system believed were committed transactions.
