[PVE-User] excessive I/O latency during CEPH rebuild

Eneko Lacunza elacunza at binovo.es
Tue Oct 28 16:13:53 CET 2014


Hi Adam,

I suggest to set noout before removing the drive, so that cluster 
doesn't start to rebalance. Then put in the new disk and in it; then 
unset noout.

That way you just get the network traffic to complete the data in that 
new disk (copy recover), and no rebalancing.

On 28/10/14 16:05, Adam Thompson wrote:
> On 14-10-28 10:03 AM, Adam Thompson wrote:
>> I'm seeing ridiculous I/O latency after out'ing and re-in'ing a disk 
>> in the CEPH array; the OSD monitor tab shows two OSDs (i.e. disks) 
>> having latency above 10msec - they're both in the 200ms range - but 
>> reading a single uncached sector from a virtual disk takes >10sec.
>>
>> It's bad enough that all my virtualized DNS servers are timing out 
>> and this, of course, directly impacts service.
>>
>> During normal (non-rebuild, non-rebalance) operations, CEPH is not 
>> terribly fast to write, but delivers acceptable read speeds.
>>
>> Where do I start looking for problems?  Are there any knobs I should 
>> be tweaking for CEPH?
>>
>
> A related question: to proactively replace a disk, I'm doing 
> Stop->Out->Remove / swap disk / Create OSD.  Is that a viable 
> procedure?  Other than the rebuild I/O starving regular reads, it 
> seems to be working...
>


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