Re: Where is the performance bottleneck?

[Holger Kiehl <Holger.Kiehl@dwd.de>]

On Wed, 31 Aug 2005, Jens Axboe wrote:

> On Wed, Aug 31 2005, Holger Kiehl wrote:
> > > # ./oread /dev/sdX
> > >
> > > and it will read 128k chunks direct from that device. Run on the same
> > > drives as above, reply with the vmstat info again.
> > Using kernel 2.6.12.5 again, here the results:
>
> [snip]
>
> Ok, reads as expected, like the buffered io but using less system time.
> And you are still 1/3 off the target data rate, hmmm...
>
> With the reads, how does the aggregate bandwidth look when you add
> 'clients'? Same as with writes, gradually decreasing per-device
> throughput?
I performed the following tests with this command:

   dd if=/dev/sd?1 of=/dev/null bs=256k count=78125

Single disk tests:

   /dev/sdc1 74.954715 MB/s
   /dev/sdg1 74.973417 MB/s

Following disks in parallel:

   2 disks on same channel
   /dev/sdc1 75.034191 MB/s
   /dev/sdd1 74.984643 MB/s

   3 disks on same channel
   /dev/sdc1 75.027850 MB/s
   /dev/sdd1 74.976583 MB/s
   /dev/sde1 75.278276 MB/s

   4 disks on same channel
   /dev/sdc1 58.343166 MB/s
   /dev/sdd1 62.993059 MB/s
   /dev/sde1 66.940569 MB/s
   /dev/sdd1 70.986072 MB/s

   2 disks on different channels
   /dev/sdc1 74.954715 MB/s
   /dev/sdg1 74.973417 MB/s

   4 disks on different channels
   /dev/sdc1 74.959030 MB/s
   /dev/sdd1 74.877703 MB/s
   /dev/sdg1 75.009697 MB/s
   /dev/sdh1 75.028138 MB/s

   6 disks on different channels
   /dev/sdc1 49.640743 MB/s
   /dev/sdd1 55.935419 MB/s
   /dev/sde1 58.795241 MB/s
   /dev/sdg1 50.280864 MB/s
   /dev/sdh1 54.210705 MB/s
   /dev/sdi1 59.413176 MB/s

So this looks different from writting, only as of four disks does the
performance begin to drop.

I just noticed, did you want me to do these test with the oread program?

Thanks,
Holger

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