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Re: Slow PITR restore

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On Wed, 12 Dec 2007 23:08:35 -0800
"Joshua D. Drake" <jd@xxxxxxxxxxxxxxxxx> wrote:

> Tom Lane wrote:
> > "Joshua D. Drake" <jd@xxxxxxxxxxxxxxxxx> writes:
> >> Tom Lane wrote:
> >>> You sure about that?  I tested CVS HEAD just now, by setting the
> >>> checkpoint_ parameters really high,
> > 
> >> ... And:
> > 
> >>> 2007-12-13 00:55:20 EST     LOG:  restored log file
> >>> "00000001000007E10000006B" from archive
> > 
> > Hmm --- I was testing a straight crash-recovery scenario, not
> > restoring from archive.  Are you sure your restore_command script
> > isn't responsible for a lot of the delay?
> 
> Now that's an interesting thought, I will review in the morning when
> I have some more IQ points back.

As promised :)... I took a look at this today and I think I found a
couple of things. It appears that once the logs are archived, the
recovery command copies the archive file to a recovery location and
then restores the file. 

If that is correct that could explain some of the latency I am seeing
here. Even with the speed of these devices, it is still a 16 MB file.
That could take 1-2 seconds to copy.

There is also the execution of pg_standby each time as the recovery
command which although I haven't timed is going to add overhead.

Based on the logs I pasted we are showing a delay of 6, 14, 3, 13, 4
and then another 6 seconds.

When are fsyncs called on the recovery process?

At these types of delays even speeding the process 2 seconds per log is
going to be significant.

Sincerely,

Joshua D. Drake


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