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Re: [dm-devel] Shell Scripts or Arbitrary Priority Callouts?



On Wed, 2009-03-25 at 12:21 -0400, John A. Sullivan III wrote:
> On Wed, 2009-03-25 at 17:52 +0200, Pasi Kärkkäinen wrote:
> > On Tue, Mar 24, 2009 at 11:41:00PM -0400, John A. Sullivan III wrote:
> > > > > Latency seems to be our key.  If I can add only 20 micro-seconds of
> > > > > latency from initiator and target each, that would be roughly 200 micro
> > > > > seconds.  That would almost triple the throughput from what we are
> > > > > currently seeing.
> > > > > 
> > > > 
> > > > Indeed :) 
> > > > 
> > > > > Unfortunately, I'm a bit ignorant of tweaking networks on opensolaris.
> > > > > I can certainly learn but am I headed in the right direction or is this
> > > > > direction of investigation misguided? Thanks - John
> > > > > 
> > > > 
> > > > Low latency is the key for good (iSCSI) SAN performance, as it directly
> > > > gives you more (possible) IOPS. 
> > > > 
> > > > Other option is to configure software/settings so that there are multiple
> > > > outstanding IO's on the fly.. then you're not limited with the latency (so much).
> > > > 
> > > > -- Pasi
> > > <snip>
> > > Ross has been of enormous help offline.  Indeed, disabling jumbo packets
> > > produced an almost 50% increase in single threaded throughput.  We are
> > > pretty well set although still a bit disappointed in the latency we are
> > > seeing in opensolaris and have escalated to the vendor about addressing
> > > it.
> > > 
> > 
> > Ok. That's pretty big increase. Did you figure out why that happens? 
> Greater latency with jumbo packets.
> > 
> > > 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.

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.

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.

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
-- 
John A. Sullivan III
Open Source Development Corporation
+1 207-985-7880
jsullivan opensourcedevel com

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