From: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | List pgsql-patches <pgsql-patches(at)postgresql(dot)org>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: [PATCHES] Infrastructure changes for recovery |
Date: | 2008-09-29 15:54:55 |
Message-ID: | 1222703695.4445.1283.camel@ebony.2ndQuadrant |
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Lists: | pgsql-hackers pgsql-patches |
On Mon, 2008-09-29 at 11:24 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> > On Mon, 2008-09-29 at 10:13 -0400, Tom Lane wrote:
> >> ... If we crash and restart, we'll have to get to the end
> >> of this file before we start letting backends in; which might be further
> >> than we actually got before the crash, but not too much further because
> >> we already know the whole WAL file is available.
>
> > Don't want to make it per file though. Big systems can whizz through WAL
> > files very quickly, so we either make it a big number e.g. 255 files per
> > xlogid, or we make it settable (and recorded in pg_control).
>
> I think you are missing the point I made above. If you set the
> okay-to-resume point N files ahead, and then the master stops generating
> files so quickly, you've got a problem --- it might be a long time until
> the slave starts letting backends in after a crash/restart.
>
> Fetching a new WAL segment from the archive is expensive enough that an
> additional write/fsync per cycle doesn't seem that big a problem to me.
> There's almost certainly a few fsync-equivalents going on in the
> filesystem to create and delete the retrieved segment files.
Didn't miss yer point, just didn't agree. :-)
I'll put it at one (1) and then wait for any negative perf reports. No
need to worry about things like that until later.
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support
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