Re: background writer, WAL and snapshot backups

Paul Tuckfield wrote:
> I really like the idea of taking a snapshot backup with postgres, using
> either volume manager or hardware splits in a disk array to get a
> physical backup.
> 
> In other, lesser database systems :) the system is structured to prevent
> problems arising from "split block" writes, meaning that though the
> database is writing an 8k block at the instant of the split, the
> intervening drivers may have written a partial block.   two ways to
> avoid this problem, as I understand, are:
> 
> 1.) freeze all block writes briefly (excluding WAL perhaps).
> Q:) I saw a patch going out to do writes in a background process. is
> this only to help write blocks before absolutely necessary, or is it to
> do *all* writes? if so, this would make adding freeze logic simple
> 
> 2.) enable logging of entire blocks to the WAL, so recovery can
> overwrite any partial blocks caught writing at the instant of split.
> Q.) Does the WAL write tuple-level changes or block-level changes.

We write entire 8k pages to WAL on first modify so we can recover
from partial 8k writes.

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