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Re: I/O on select count(*)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Matthew Wakeling <matthew(at)flymine(dot)org>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: I/O on select count(*)
Date: 2008-05-15 14:52:58
Message-ID: 4758.1210863178@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Matthew Wakeling <matthew(at)flymine(dot)org> writes:
> Hmm. That problem is what WAL full-page-writes is meant to handle, isn't 
> it? So basically, if you're telling people that WAL full-page-writes is 
> safer than partial WAL, because it avoids updating pages in-place, then 
> you shouldn't be updating pages in-place for the hint bits either. You 
> can't win!

This argument ignores the nature of the data change.  With a hint-bit
update, no data is being shuffled around, so there is no danger from a
partial page write.

A disk that leaves an individual sector corrupt would be a problem,
but I don't think that's a huge risk.  Keep in mind that disks aren't
designed to just stop dead when power dies --- they are made to be able
to park their heads before the juice is entirely gone.  I think it's
reasonable to assume they'll finish writing the sector in progress
before they start parking.

			regards, tom lane

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