[Beowulf] NFS over RDMA performance confusion

Vincent Diepeveen diep at xs4all.nl
Thu Sep 13 05:52:31 PDT 2012


On Sep 13, 2012, at 2:34 PM, Joe Landman wrote:

> On 09/13/2012 07:52 AM, holway at th.physik.uni-frankfurt.de wrote:
>
> [...]
>
>> If I set up a single machine to hammer the fileserver with IOzone  
>> I see
>> something like 50,000 IOPS but if all four machines are hammering the
>> filesystem concurrently we got it up to 180,000 IOPS.
>
> I wouldn't recommend IOzone for this sort of testing.  Its not a very
> good load generator, and it has a tendency to report things which are
> not actually seen at the hardware level.  I'd noticed this some years
> ago, when running some of our benchmark testing on these units,  
> that an
> entire IOzone benchmark completed with very few activity lights  
> going on
> the disks.  Which suggested that the test was happily entirely cached,
> and I was running completely within cache.
>
> Use fio.
>
> Second, are the disks behind the NFS/ZFS server solid state, ram disk,
> or spinning rust?
>
>> Can anyone tell me what might be the bottleneck on the single  
>> machines?
>> Why can I not get 180,000 IOPS when running on a single machine.
>
> 50k  IOPs x 16k/IOP =  819 MB/s

10 gbit TCP limit i'd guess

>
> 180k IOPs x 16k/IOP = 2949 MB/s (close to pragmatic limits on QDR)
>

40 gbit QDR yeah

the actual data goes through RDMA here and maybe through TCP using  
mysql.

> Some observations ... these don't sound like disk subsystems.  A  
> 15k RPM
> drive will give you ~300 IOPs.  To get 50k IOPs, you would need 167  
> disk
> drives per machine, operating in a best performance case scenario  
> (RAID0
> or JBOD).  To get 180k IOPs, you'd need 600x 15k RPM disks.  I am
> guessing you don't have that.
>
> Are you asking why a single machine cannot fill your QDR bandwidth?
>
> I'd recommend running traces on the individual machines to see where
> things are getting lost.  One you have the traces, post em, and see if
> people can help.
>
> -- 
> Joseph Landman, Ph.D
> Founder and CEO
> Scalable Informatics Inc.
> email: landman at scalableinformatics.com
> web  : http://scalableinformatics.com
>         http://scalableinformatics.com/sicluster
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> fax  : +1 866 888 3112
> cell : +1 734 612 4615
>
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