Re: speedup ceph / scaling / find the bottleneck

[Stefan Priebe <s.priebe@profihost.ag>]

Am 29.06.2012 17:28, schrieb Sage Weil:
> On Fri, 29 Jun 2012, Stefan Priebe - Profihost AG wrote:
>> Am 29.06.2012 13:49, schrieb Mark Nelson:
>>> I'll try to replicate your findings in house.  I've got some other
>>> things I have to do today, but hopefully I can take a look next week. If
>>> I recall correctly, in the other thread you said that sequential writes
>>> are using much less CPU time on your systems?
>>
>> Random 4k writes: 10% idle
>> Seq 4k writes: !! 99,7% !! idle
>> Seq 4M writes: 90% idle
>
> I take it 'rbd cache = true'?
Yes

> It sounds like librbd (or the guest file
> system) is coalescing the sequential writes into big writes.  I'm a bit
> surprised that the 4k ones have lower CPU utilization, but there are lots
> of opportunity for noise there, so I wouldn't read too far into it yet.
90 to 99,7 is OK the 9% goes to flush, kworker and xfs processes. It was 
the overall system load. Not just ceph-osd.

>>>   Do you see better scaling in that case?
>>
>> 3 osd nodes:
>> 1 VM:
>> Rand 4k writes: 7000 iops
<-- this one is WRONG! sorry it is 14100 iops


>> Seq 4k writes: 19900 iops
>>
>> 2 VMs:
>> Rand 4k writes: 6000 iops each
>> Seq 4k writes: 4000 iops VM 1
>> Seq 4k writes: 18500 iops VM 2
>>
>>
>> 4 osd nodes:
>> 1 VM:
>> Rand 4k writes: 14400 iops      <------ ????
>
> Can you double-check this number?
Triple checked BUT i see the the Rand 4k writes with 3 osd nodes was 
wrong. Sorry.

>> Seq 4k writes: 19000 iops
>>
>> 2 VMs:
>> Rand 4k writes: 7000 iops each
>> Seq 4k writes: 18000 iops each
>
> With the exception of that one number above, it really sounds like the
> bottleneck is in the client (VM or librbd+librados) and not in the
> cluster.  Performance won't improve when you add OSDs if the limiting
> factor is the clients ability to dispatch/stream/sustatin IOs.  That also
> seems concistent with the fact that limiting the # of CPUs on the OSDs
> doesn't affect much.
ACK

> Aboe, with 2 VMs, for instance, your total iops for the cluster doubled
> (36000 total).  Can you try with 4 VMs and see if it continues to scale in
> that dimension?  At some point you will start to saturate the OSDs, and at
> that point adding more OSDs should show aggregate throughput going up.
 From where did you get that value? It scales to VMs on some points but 
it does not scale with OSDs.

Stefan