| From: | Jeff Davis <pgsql(at)j-davis(dot)com> |
|---|---|
| To: | Hannes Dorbath <light(at)theendofthetunnel(dot)de> |
| Cc: | pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: 8.3 synchronous_commit |
| Date: | 2008-01-21 22:25:30 |
| Message-ID: | 1200954330.10057.529.camel@dogma.ljc.laika.com |
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| Thread: | |
| Lists: | pgsql-performance |
On Mon, 2008-01-21 at 22:55 +0100, Hannes Dorbath wrote:
> I might completely misunderstand this feature. Shouldn't
> "synchronous_commit = off" improve performance?
>
> Whatever I do, I find "synchronous_commit = off" to degrade performance.
>
> Especially it doesn't like the CFQ I/O scheduler, it's not so bad with
> deadline. Synthetic load like
>
The CFQ scheduler is bad for performance in the tests that I have run.
When I have a chance I'll put together some tests to try to demonstrate
that.
The reason it may be bad in your case is if you have many backends
commit many transactions asynchronously, and then the WAL writer tries
to make those transactions durable, CFQ might think that the WAL writer
is "unfairly" using a lot of I/O. This is just speculation though.
Regards,
Jeff Davis
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