PostgreSQL – How to Move Data to a Different Drive

See the fine manual. If my advice conflicts with its' in any way, it's right.

1. A sync isn't a bad idea, unless your copy tool fsync()s each WAL file it writes and the directory it's in before copying the next one. An incomplete last WAL file doesn't matter much; at worst, you just delete it. Pg will generally choke on an incomplete WAL - though there's no checksumming done, so you could be really unlucky and have it try to apply garbage data that by sheer insane chance happened to look like real WAL records. In your position I'd be syncing the volume before a snapshot to make sure any unwritten dirty buffers in RAM hit the file system image on disk. A freeze would help avoid messy but non-fatal partially written WALs, so it's not a terrible idea but not vital. What's vital is to have an undamaged timeline up until the point of recovery. Personally, I write my WALs to a temporary file name and rename them to their final name only once fully copied; if you do this, you don't need to freeze.

2. Sounds correct. A live snapshot is just like doing a plug pull test on a live system with write-through caching. Your database should recover fine when restored from a live snapshot, same as after plug-pull. I'd recommend that you automate tests of restores from snapshots. (Note: A snapshot restore test is not a complete substitute for plug pull testing because it doesn't account for possible disk, raid controller, etc write caching). Not only the config files and the dump, but the database its self should be fine after your snapshot. Consider syncing the volume before the snapshot to make sure all the dump data etc has actually hit disk.
   
   2a. Might save some disk space. Little difference otherwise. You'll get to keep the snapshots a lot longer without all the churn of the live database on them.

3. Why even snapshot your code volume? A plain file level copy may well be just fine. Certainly a live snapshot should be.

4. This is not a solid backup scheme. It fails in one critical area: There is no restore testing and validation being performed. You should always test your backups on a regular basis to make sure you can really restore them.

   Personally, I recommend that you use WAL shipping, or send database dumps, to a different host, preferably one not on Amazon EC2 or at least in a different region. This host should perform automated restore tests, send reports to you of the results, and should also be checked manually.

   While your snapshots (containing dumps) will be on S3, and will be safe there, that doesn't mean they'll be accessible when you need them urgently. Amazon's durability claims are reassuring, but your data can still be safe and completely inaccessible to you during a badly timed outage of the S3 service.

The documentation you cited says it all, but I wouldn't blame you if you want to try to verify the claims of the vendor regarding snapshots taken at the same time. Perhaps a way of uncovering something could be to stress test the WAL system more specifically.

e.g. In addition to your pgbench-based tests, try adding random calls to pg_switch_xlog() to force log rotation, shorter and longer checkpoint intervals (shortening and lengthening checkpoint_timeout and checkpoint_timeout) and even using small or large wal file sizes.

Unless there's something I'm missing, for your snapshots not taken at the same time, I would attribute your recovered DBs perhaps to a bit of lucky timing. In the last case, imagine you took your log snapshot while the current xlog location was, say, 0/A1C0FFEE. Then you have 3 minutes of particularly heavy load on the system, that causes a full cycle through the WAL files, and your DB is now at 0/DEADBEEF when the data snapshot is taken. When you attempt to restore, the WAL files being written to at the time of the data snapshot are long gone, and recovery will fail.