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



Bruno Wolff III wrote:
On Mon, Dec 11, 2006 at 12:44:40 -0500,
  Ric Wheeler <ric 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.

Glad to see that it is useful.

Thanks really go out to Jens Axboe and Chris Mason for getting this all to work correctly in the first place ;-)

As I mentioned in our private email exchange, you should see better performance with the write barrier on vs write cache disabled for some scenarios (large file writes and using data journal mode for small files are the two cases that I have noticed).

ric



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