Re: Load Distributed Checkpoints, final patch

Added to TODO:

* Test to see if calling PreallocXlogFiles() from the background writer
will help with WAL segment creation latency

http://archives.postgresql.org/pgsql-patches/2007-06/msg00340.php

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Tom Lane wrote:
> Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
> > Here's latest revision of Itagaki-sans Load Distributed Checkpoints patch:
>
> Applied with some minor revisions to make some of the internal APIs a
> bit cleaner; mostly, it seemed like a good idea to replace all those
> bool parameters with a flag-bits approach, so that you could have
> something like "CHECKPOINT_FORCE | CHECKPOINT_WAIT" instead of
> "false, true, true, false" ...
>
> For the moment I removed all the debugging elog's in the patch.
> We still have Greg Smith's checkpoint logging patch to look at
> (which I suppose needs adjustment now), and that seems like the
> appropriate venue to consider what to put in.
>
> Also, the question of redesigning the bgwriter's LRU scan is
> still open. I believe that's on Greg's plate, too.
>
> One other closely connected item that might be worth looking at is the
> code for creating new future xlog segments (PreallocXlogFiles). Greg
> was griping upthread about xlog segment creation being a real
> performance drag. I realized that as we currently have it set up, the
> checkpoint code is next to useless for high-WAL-volume installations,
> because it only considers making *one* future XLOG segment. Once you've
> built up enough XLOG segments, the system isn't too bad about recycling
> them, but there will be a nasty startup transient where foreground
> processes have to stop and make the things. I wonder whether it would
> help if we (a) have the bgwriter call PreallocXlogFiles during its
> normal loop, and (b) back the slop in PreallocXlogFiles way off, so that
> it will make a future segment as soon as we start using the last
> existing segment, instead of only when we're nearly done. This would at
> least make it more likely that the bgwriter does the work instead of a
> foreground process. I'm hesitant to go much further than that, because
> I don't want to bloat the minimum disk footprint for low-volume
> installations, but the minimum footprint is really 2 xlog files anyway...
>
> regards, tom lane
>
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