Re: UPDATEDs slowing SELECTs in a fully cached database

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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