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Isn't there a risk with EBS snapshots that the snapshot of a live instance could have been taken while your db engine was in the middle of a transaction and leave the data in the newly spun instance in an inconsistent state?

Is it that EBS snapshots are engineered to prevent this? Or just that it's not likely to happen in practice?

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Yes, there is--we take all of our snapshots from a slave, and we stop the slave before taking a snapshot, then XFS-freeze all drives, then take the snapshot, to ensure it's consistent.

Are EBS snapshots not block-level atomic? In theory you should get a PITR image without stopping anything, assuming that:

1) The file system correctly orders or journals operations (I'm not familiar with XFS, but this is the case with FFS2/FreeBSD, ZFS, ext3/4 journaling, etc).

2) The database system correctly orders or journals operations, and properly fsync(s) to disk (which postgreSQL does)

Of course, there's no harm to an abundance of caution with something like this.

They are, but we software-RAID our EBS drives to get better write throughput, and we put the Write-Ahead Logs (WALs) on a different RAID from the main database, so when you have both of those going on, you need something else to atomically snapshot our PG databases.

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