Re: IOPS higher than expected on randwrite, direct=1 tests

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* Jens Axboe <jaxboe@xxxxxxxxxxxx> wrote:
> On 2010-11-10 19:58, Sebastian Kayser wrote:
> > Interim update. Exported the whole 2TB disk as a LUN, mkfs.ext3'd it and
> > set size=100g in fio's configuration. Also set runtime=1800, re-started
> > the test and could observe ~80 IOPS ... my dear heart was jumping with
> > joy :)
> > 
> > However, a few minutes into the test, IOPS started to increase steadily
> > and by now have again reached (non-bursty) regions that don't seem
> > plausible for a single 7.2K SATA disk.
> > 
> > root@ubuntu-804-x64:~# ./fio --section=iscsi patterns.fio 
> > iscsi: (g=0): rw=randwrite, bs=4K-4K/4K-4K, ioengine=sync, iodepth=1
> > Starting 1 process
> > iscsi: Laying out IO file(s) (1 file(s) / 102400MB)
> > Jobs: 1 (f=1): [w] [48.4% done] [0K/986K /s] [0/240 iops] [eta 15m:28s] 
> A 7200RPM drive will spin around 120 times per second, that yields an
> average rotational latency of 8.3 msecs. For truly random IO, rotational
> latency will dominate the seek and the average wait-for-platter-spin
> will be half that, so 4.17 msecs. That gives us about 240 IOPS.
> So your results don't seem all that out of whack. What are your reasons
> for expecting ~80 IOPS?

I always learnt that:

  disk latency = avg. seek time + (rotational delay / 2)
                 + negligible amount of transfer time

Where "seek time" is not neglible. The Hitachi deskstar which is built
into our storage box [1] doesn't come with seek time specs, but taking a
similar enterprise model [2] Hitachi specifies an average seek time of
8.2ms - about twice as much as the average rotational delay component of
4.17ms.  Adding the two together gives me 8.2 + 4.17 = 12.37ms per IO.
Which in turn gives me 1000 / 12.37 = 80.84 IOPS.


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