Re: Advice configuring ServeRAID 8k for performance

On Mon, Aug 16, 2010 at 01:46:21PM -0400, Greg Smith wrote:
> Scott Carey wrote:
> >This is because an fsync on ext3 flushes _all dirty pages in the file system_ to disk, not just those for the file
beingfsync'd. 
> >One partition for WAL, one for data.  If using ext3 this is
> >essentially a performance requirement no matter how your array is
> >set up underneath.
>
> Unless you want the opposite of course.  Some systems split out the
> WAL onto a second disk, only to discover checkpoint I/O spikes
> become a problem all of the sudden after that.  The fsync calls for
> the WAL writes keep the write cache for the data writes from ever
> getting too big.  This slows things down on average, but makes the
> worst case less stressful.  Free lunches are so hard to find
> nowadays...
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.)...

Andres