Re: [PATCHES] Infrastructure changes for recovery

Simon Riggs <simon@2ndQuadrant.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