Re: I/O on select count(*)

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