| From: | "Simon Riggs" <simon(at)2ndquadrant(dot)com> |
|---|---|
| To: | "Pavan Deolasee" <pavan(dot)deolasee(at)gmail(dot)com> |
| Cc: | "Heikki Linnakangas" <heikki(at)enterprisedb(dot)com>, "PostgreSQL-development" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Piggybacking vacuum I/O |
| Date: | 2007-01-24 10:43:40 |
| Message-ID: | 1169635420.3776.624.camel@silverbirch.site |
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| Lists: | pgsql-hackers |
On Wed, 2007-01-24 at 09:32 +0530, Pavan Deolasee wrote:
> On a typical desktop class 2 CPU Dell machine, we have seen pgbench
> clocking more than 1500 tps. That implies CLOG would get filled up in
> less
> than 262144/1500=174 seconds. VACUUM on accounts table takes much
> longer to trigger.
You assume that all of the top level transactions have no
subtransactions. On that test, subtransactions are in use because of the
EXCEPTION clause in the PL/pgSQL used. That should at least double the
number of Xids.
> So
> most of the 636528 reads in the next 55 minutes can be attributed to
> VACUUM.
A similar argument might also be applied to subtrans, so a similar
investigation seems worthwhile. Subtrans has space for less Xids than
clog, BTW.
OTOH, I do think that 99% of that will not cause I/O.
--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com
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