Re: wal_level=archive gives better performance than minimal - why?

On 16.1.2012 23:35, Greg Smith wrote:
> On 01/12/2012 06:17 PM, Tomas Vondra wrote:
>> I've run a series fo pgbench benchmarks with the aim to see the effect
>> of moving the WAL logs to a separate drive, and one thing that really
>> surprised me is that the archive log level seems to give much better
>> performance than minimal log level.
>
> How repeatable is this?  If you always run minimal first and then
> archive, that might be the actual cause of the difference.  In this
> situation I would normally run this 12 times, with this sort of pattern:
>
> minimal
> minimal
> minimal
> archive
> archive
> archive
> minimal
> minimal
> minimal
> archive
> archive
> archive
>
> To make sure the difference wasn't some variation on "gets slower after
> each run".  pgbench suffers a lot from problems in that class.

AFAIK it's well repeatable - the primary goal of the benchmark was to
see the benefir of moving the WAL to a separate device (with various WAL
levels and device types - SSD and HDD).

I plan to rerun the whole thing this week with a bit more details logged
to rule out basic configuration mistakes etc.

Each run is completely separate (rebuilt from scratch) and takes about 1
hour to complete. Each pgbench run consists of these steps

  1) rebuild the data from scratch
  2) 10-minute warmup (read-only run)
  3) 20-minute read-only run
  4) checkpoint
  5) 20-minute read-write run

and the results are very stable.

Tomas