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Re: [PATCHES] Infrastructure changes for recovery

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
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 15:24:08
Message-ID: 12423.1222701848@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
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.

			regards, tom lane

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