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Re: I/O increase after upgrading to 8.3.5

From: Greg Smith <gsmith(at)gregsmith(dot)com>
To: Alexander Staubo <alex(at)bengler(dot)no>
Cc: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: I/O increase after upgrading to 8.3.5
Date: 2009-02-15 17:35:04
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Sat, 14 Feb 2009, Alexander Staubo wrote:

> Are there any statistics, either in PostgreSQL proper or in the OS, that 
> I can use as metrics to guide the tuning? For example, is there anything 
> in pg_stat_bgwriter that can help me tune the bgwriter_lru_* settings? goes 
over this topic, with "Appendix B: pg_stat_bgwriter sample analysis" 
covering a look at what to do based on a pg_stat_bgwriter snapshot.

> Do transactions that only contain query statements end up writing
> entries to the WAL when they commit?

You need a transactions XID before you can write to the WAL, and quoting 
from the transaction management docs:

"Transactions and subtransactions are assigned permanent XIDs only when/if 
they first do something that requires one --- typically, 
insert/update/delete a tuple"

The main thing that will cause writes even when reading are hint bit 
updates, which have been mentioned here already. has an intro to that topic.

* Greg Smith gsmith(at)gregsmith(dot)com Baltimore, MD

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Subject: Re: I/O increase after upgrading to 8.3.5
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