On Fri, Mar 27, 2009 at 03:03:35AM -0400, John A. Sullivan III wrote: > > > > > > > The once piece which is still a mystery is why using four targets on > > > > four separate interfaces striped with dmadm RAID0 does not produce an > > > > aggregate of slightly less than four times the IOPS of a single target > > > > on a single interface. This would not seem to be the out of order SCSI > > > > command problem of multipath. One of life's great mysteries yet to be > > > > revealed. Thanks again, all - John > > > > > > Hmm.. maybe the out-of-order problem happens at the target? It gets IO > > > requests to nearby offsets from 4 different sessions and there's some kind > > > of locking or so going on? > > Ross pointed out a flaw in my test methodology. By running one I/O at a > > time, it was literally doing that - not one full RAID0 I/O but one disk > > I/O apparently. He said to truly test it, I would need to run as many > > concurrent I/Os as there were disks in the array. Thanks - John > > ><snip> > Argh!!! This turned out to be alarmingly untrue. This time, we were > doing some light testing on a different server with two bonded > interfaces in a single bridge (KVM environment) going to the same SAM we > used in our four port test. > Is the SAN also using bonded interfaces? > For kicks and to prove to ourselves that RAID0 scaled with multiple I/O > as opposed to limiting the test to only single I/O, we tried some actual > file transfers to the SAN mounted in sync mode. We found concurrently > transferring two identical files to the RAID0 array composed of two > iSCSI attached drives was 57% slower than concurrently transferring the > files to the drives separately. In other words, copying file1 and file2 > concurrently to RAID0 took 57% longer than concurrently copying file1 to > disk1 and file2 to disk2. > Hmm.. I wonder why that happens.. > We then took a little different approach and used disktest. We ran two > concurrent sessions with -K1. In one case, we ran both sessions to the > 2 disk RAID0 array. The performance was significantly less again, than > running the two concurrent tests against two separate iSCSI disks. Just > to be clear, these were the same disks as composed the array, just not > grouped in the array. > There has to be some logical explanation to this.. > Even more alarmingly, we did the same test using multipath multibus, > i.e., two concurrent disktest with -K1 (both reads and rights, all > sequential with 4K block sizes). The first session completely starved > the second. The first one continued at only slightly reduced speed > while the second one (kicked off just as fast as we could hit the enter > key) received only roughly 50 IOPS. Yes, that's fifty. > > Frightening but I thought I had better pass along such extreme results > to the multipath team. Thanks - John Hmm, so you had mpath0 and mpath1, and you ran disktest against both, at the same time? -- Pasi -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
