Message-ID: <4E1A31AB.9010201@postnewspapers.com.au>
Date: Mon, 11 Jul 2011 07:11:39 +0800
From: Craig Ringer <craig@postnewspapers.com.au>
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To: pgsql-performance@postgresql.org
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database
References: <4E16479D.3050605@yahoo.com> <4E1A0CBC.6090009@yahoo.com>
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On 11/07/2011 4:34 AM, lars wrote:
> I have since moved the WAL to its own EBS volume (ext4, data=writeback)
> to make it easier to monitor IO.
> The times where the SELECTs slow down coincide with heavy write traffic
> to the WAL volume.

In theory, UPDATEs shouldn't be blocking or slowing SELECTs. Whether 
that holds up to the light of reality, real-world hardware, and software 
implementation detail, I really don't know. I avoided responding to your 
first mail because I generally work with smaller and less performance 
critical databases so I haven't accumulated much experience with 
fine-tuning.

If your SELECTs were slower *after* your UPDATEs I'd be wondering if 
your SELECTs are setting hint bits on the pages touched by the UPDATEs. 
See: http://wiki.postgresql.org/wiki/Hint_Bits . It doesn't sound like 
that's the case if the SELECTs are slowed down *during* a big UPDATE 
that hasn't yet committed, though.

Could it just be cache pressure - either on shm, or operating system 
disk cache? All the dirty buffers that have to be flushed to WAL and to 
the heap may be evicting cached data your SELECTs were benefitting from. 
Unfortunately, diagnostics in this area are ... limited ... though some 
of the pg_catalog views 
(http://www.postgresql.org/docs/9.0/static/catalogs.html) may offer some 
information.

--
Craig Ringer

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