Message-ID: <5265B562.3040407@fuzzy.cz>
Date: Tue, 22 Oct 2013 01:14:42 +0200
From: Tomas Vondra <tv@fuzzy.cz>
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To: pgsql-performance@postgresql.org
Subject: Re: Hot Standby performance issue
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On 22.10.2013 00:59, sparikh wrote:
> Yes, Expalin without Analyze is taking long. It is weird. In the 
> pg_stat_activity Explain was the only query running. So server was
> almost idle. Using New relic interface I checked CPU was almost idle
> - around 10-20%. There were some IO activity - around 40-50%.
> 
> I forgot to mention before I could run perf on command line even with
> root permission. It says command not found. May be utility is not
> installed or not enabled.

Obviously you need to install it ... maybe ask your sysadmin to do that.

> I have attached the snapshot of vmstat while explain was running in 
> background. vmstat.txt 
> <http://postgresql.1045698.n5.nabble.com/file/n5775349/vmstat.txt>

The vmstat clearly shows that ~1 CPU is waiting on I/O. Hmm, I'm really
wondering what's going on here - I can't think of a case where this
would happen with a plain EXPLAIN ...

We really need the perf results. Or try to run strace, maybe it'll give
more info about which files it accesses.

> Do you suggest if I remove all the data files from /data/base folder
> of standby and again rebuild using rsync from primary ? do you see
> any issues there.? This is just to rule out any fragmentation on
> standby side.

The EXPLAIN really should not do much I/O. I doubt it has anything to do
with fragmentation, so I doubt this is going to help.

> Our sys admin is planning to run fsck sometime today or tomorrow.

OK. Which filesystem do you use, btw?

Tomas


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