On 5 February 2018 at 04:33, Jérôme Charaoui <jcharaoui@xxxxxxxxxxxxxxxxxx> wrote: > > I've been running some tests on a bunch of disks lately and came across > a strange result that I'm struggling to explain. > > I built a 20-device raid10 md array with SSDs and ran several fio tests > on it. The randwrite test really stands out because the IOPS starts out > fairly low, around 800 IOPS, but within a few seconds climbs up to about > 70K IOPS, which is nearly three times higher than the IOPS I'm getting > with the randread test (25K). > > I tried disabling the drive caches using hdparm -W 0, and also disabling > the internal write-intent bitmap on the md array, but the results are > the same. You didn't include the fio job you're using (see https://github.com/axboe/fio/blob/fio-3.3/REPORTING-BUGS ) so it's impossible to say anything too useful. I'd check whether a solo SSD by itself exhibits similar variation. It's also worth noting the order you read data back compared to how it was written can impact an SSD (see http://codecapsule.com/2014/02/12/coding-for-ssds-part-5-access-patterns-and-system-optimizations/ ) but I wouldn't have thought it should be to such a degree. Perhaps http://vger.kernel.org/vger-lists.html#linux-raid or http://vger.kernel.org/vger-lists.html#linux-block might give better replies... -- Sitsofe | http://sucs.org/~sits/ -- To unsubscribe from this list: send the line "unsubscribe fio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
