Re: Advice configuring ServeRAID 8k for performance

On Mon, Aug 16, 2010 at 04:13:22PM -0400, Greg Smith wrote:
> Andres Freund wrote:
> >Or use -o sync. Or configure a ridiciuosly low dirty_memory amount
> >(which has a problem on large systems because 1% can still be too
> >much. Argh.)...
>
> -o sync completely trashes performance, and trying to set the
> dirty_ratio values to even 1% doesn't really work due to things like
> the "congestion avoidance" code in the kernel. If you sync a lot
> more often, which putting the WAL on the same disk as the database
> accidentally does for you, that works surprisingly well at avoiding
> this whole class of problem on ext3. A really good solution is
> going to take a full rewrite of the PostgreSQL checkpoint logic
> though, which will get sorted out during 9.1 development. (cue
> dramatic foreshadowing music here)
-o sync works ok enough for the data partition (surely not the wal) if you make the
background writer less aggressive.

But yes. A new checkpointing logic + a new syncing logic
(prepare_fsync() earlier and then fsync() later) would be a nice
thing. Do you plan to work on that?

Andres