Re: Pre-page images in WAL

> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > I was wondering if we should disable the writing of pre-page images into
> > WAL if the user has turned off fsync?
> 
> I'm worried about what vulnerabilities that would create.
> 
> Historically we've always defined "fsync off" to mean "I trust my
> kernel, hardware, and power supply ... but not necessarily Postgres
> itself".  In a Postgres crash, even with fsync off, you are not supposed
> to lose any committed transactions, so long as the kernel and hardware
> stay up.
> 
> In the brave new world of WAL, Postgres does not flush dirty buffers to
> disk at transaction commit, relying on WAL to clean up if a database or
> system failure occurs.  If we don't log page images to WAL then I think
> there's a hole here wherein a Postgres crash can lose data even though
> no failure of the surrounding OS occurs.  Maybe it's safe, but I'm not
> convinced.

I understand.  The only reason I mentioned this is because I thought
pre-page WAL was only to cache partially written disk pages.

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
853-3000+  If your life is a hard drive,     |  830 Blythe Avenue +  Christ can be your backup.        |  Drexel Hill,
Pennsylvania19026