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We're running on EBS volumes on EC2. We're interested in leveraging EBS snapshotting for backups. However, does this mean we'd need to ensure our pg_xlog is on the same EBS volume as our data?

(I believe) the usual reasoning for separating pg_xlog onto a separate volume is for performance. However, if they are on different volumes, the snapshots may be out of sync.

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Having the transaction log (pg_xlog) on separate storage can increase write performance on most RDBMS platforms. If yours is a (concurrent) write heavy application, you may see a performance benefit.

If the snapshots are not synchronous however, you could lose data from uncomitted transactions in the event of an unclean restart. I suppose in certain situations a pg_resetxlog could be required after a failure.

You could look into WAL archiving, or even techniques similar to ZFS snapshotting if EBS snapshots have the same effective purpose.

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