Re: Patch: Write Amplification Reduction Method (WARM)

On Tue, Mar 21, 2017 at 04:04:58PM -0400, Bruce Momjian wrote:
> On Tue, Mar 21, 2017 at 04:56:16PM -0300, Alvaro Herrera wrote:
> > Bruce Momjian wrote:
> > > On Tue, Mar 21, 2017 at 04:43:58PM -0300, Alvaro Herrera wrote:
> > > > Bruce Momjian wrote:
> > > >
> > > > > I don't think it makes sense to try and save bits and add complexity
> > > > > when we have no idea if we will ever use them,
> > > >
> > > > If we find ourselves in dire need of additional bits, there is a known
> > > > mechanism to get back 2 bits from old-style VACUUM FULL. I assume that
> > > > the reason nobody has bothered to write the code for that is that
> > > > there's no *that* much interest.
> > >
> > > We have no way of tracking if users still have pages that used the bits
> > > via pg_upgrade before they were removed.
> >
> > Yes, that's exactly the code that needs to be written.
>
> Yes, but once it is written it will take years before those bits can be
> used on most installations.

Actually, the 2 bits from old-style VACUUM FULL bits could be reused if
one of the WARM bits would be set when it is checked. The WARM bits
will all be zero on pre-9.0. The check would have to be checking the
old-style VACUUM FULL bit and checking that a WARM bit is set.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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