On Fri, Aug 26, 2011 at 10:46:33AM +0200, Tomas Vondra wrote:
> Hmmm, let me explain what led me to this patch - right now I'm doing a
> comparison of filesystems with various block sizes (both fs and db
> blocks). I've realized that the db block size significantly influences
> frequency of checkpoints and amount of data to write, so I'm collecting
> data from pg_stat_bgwriter too. The benchmark goes like this
>
> 1. collect pg_stat_bgwriter stats
> 2. run pgbench for 10 minutes
> 3. collect pg_stat_bgwriter stats (to compute difference with (1))
> 4. kill the postmaster
>
> The problem is that when checkpoint stats are collected, there might be a
> checkpoint in progress and in that case the stats are incomplete. In some
> cases (especially with very small db blocks) this has significant impact
> because the checkpoints are less frequent.
Could you remove this hazard by adding a step "2a. psql -c CHECKPOINT"?
--
Noah Misch http://www.2ndQuadrant.com/
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