fsync, ext3, raid (md) 1, write barriers and PATA caching

[Bruno Wolff III]

On Mon, Dec 11, 2006 at 12:44:40 -0500,
  Ric Wheeler <ric at emc.com> wrote:
> 
> I will send you the test code that I use & some test runs,

I tried out Ric's test code and it does appear that the barrier is working
under fc5 using software raid. For the quick test I didn't have an idle
system. If I ran with either no barrier option and write caching enabled
I saw very roughly a 10x speed up. The test seemed to show running with
write caching disabled was about 20% faster than with just using barriers.
My theory is that because there was a lot of other disk activity, the cache
flushes forced by using barriers was writing a lot of disk blocks from
outside the test making it report slower numbers, while in theory my system
throughput was actually better.
Tonight I can try the test on a system without a lot of disk activity and
see if that makes much difference.

Thanks Ric.