Re: Slow Query / Check Point Segments

From: John R Pierce <pierce(at)hogranch(dot)com>
To: Greg Smith <greg(at)2ndquadrant(dot)com>
Cc: pgsql-general(at)postgresql(dot)org
Subject: Re: Slow Query / Check Point Segments
Date: 2010-01-23 09:58:51
Message-ID: 4B5AC85B.6070303@hogranch.com
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Greg Smith wrote:
> My guess is that there's something wrong with your config such that
> writes followed by fsync are taking longer than they should. When I
> see "sync=0.640 s" into a SAN where that sync operation should be near
> instant, I'd be looking for issues in the ZFS intent log setup, how
> much data you've setup ZFS to write cache, and the write cache policy
> on the SAN hardware. There's something weird going on here, that sync
> should be near instant on your hardware with such a small write volume.
>
> Unfortunately, you may have to take the server down to find out
> exactly what's going on, which doesn't sound practical for your
> environment. (And people wonder why I have near religious zeal about
> testing disk hardware before systems go into production)

there's a further complication. this system is deep in southeast asia
on a rather slow and saturated network connection such that trying to
figure out stuff about it is challenging at best. I know the database
has a lot of write volume overall, and its only one of several databases
running in different zones on the server. I know nothing about the SAN,
I suspect its a EMC Symmetrix of some sort. Probably a generation or
two behind latest. The operations people are used to running large
oracle databases.

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