Re: 16-bit page checksums for 9.2

On Saturday, December 24, 2011 05:01:02 PM Simon Riggs wrote:
> On Sat, Dec 24, 2011 at 3:54 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On Saturday, December 24, 2011 03:46:16 PM Tom Lane wrote:
> >> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> >> > After the various recent discussions on list, I present what I believe
> >> > to be a working patch implementing 16-but checksums on all buffer
> >> > pages.
> >>
> >> I think locking around hint-bit-setting is likely to be unworkable from
> >> a performance standpoint. I also wonder whether it might not result in
> >> deadlocks.
> >
> > Why don't you use the same tricks as the former patch and copy the
> > buffer, compute the checksum on that, and then write out that copy (you
> > can even do both at the same time). I have a hard time believing that
> > the additional copy is more expensive than the locking.
>
> We would copy every time we write, yet lock only every time we set hint
> bits.
Isn't setting hint bits also a rather frequent operation? At least in a well-
cached workload where most writeout happens due to checkpoints.

> If that option is favoured, I'll write another version after Christmas.
Seems less complicated (wrt deadlocking et al) to me. But I havent read your
patch, so I will shut up now ;)

Andres