Re: Issues with two-server Synch Rep

On Mon, Oct 11, 2010 at 7:16 PM, Jeff Davis <pgsql(at)j-davis(dot)com> wrote:

> On Mon, 2010-10-11 at 16:07 -0400, Robert Haas wrote:
> > >> It would be far better if we could decouple master cleanup from
> > >> standby cleanup, so that only the machine that actually has the old
> > >> query gets bloated. However, no one seems excited about writing that
> > >> code.
> > >
> > > That doesn't seem just a matter of code, it seems like a major design
> > > conflict.
> >
> > Yes. I had the idea of trying to fix this by allowing the standby to
> > retain old versions of entire pages that got cleaned up on the master,
> > until the transactions that might want to read the old pages were
> > gone. But that may be prohibitively difficult, not sure.
>
> I think you'd end up having a notion of a snapshot of block information
> (like a FS with snapshots) inside of postgres.
>
> Sounds like a lot of complexity to me, and the only benefit I see is
> moving bloat from the primary to the standby. Granted, that would be
> nice, but I would expect some costs aside from just the complexity.
>

I had this idea when HS was being initially implemented, and I saw two
problems with it.

.) Identifying block versions in buffers. Our XID based MVCC like scheme
would work, but we already have a block header which we would want to trim.

.) Effectively every block cleanup will have to create a copy of the block
because more often than not there's at least one transaction in the system,
and when it asks, we need to hand it the old version.

Regards,
--
gurjeet.singh
@ EnterpriseDB - The Enterprise Postgres Company
http://www.EnterpriseDB.com

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