The drive in my test machine at the moment is a Kingston KC300 - not the most “DC” class but with good DWPD rating and worked well for us (beware of newer revisions though!).
Throttling is not the problem, I am either saturating my SATA link capacity or the drive (or both), either way CFQ is what completely corks it up...
> On 23 Jun 2015, at 14:21, Eneko Lacunza <elacunza@xxxxxxxxx> wrote:
>
> Hi Jan,
>
> What SSD model?
>
> I've seen SSDs work quite well usually but suddenly give a totally awful performance for some time (not those 8K you see though).
>
> I think there was some kind of firmware process involved, I had to replace the drive with a serious DC one.
>
> El 23/06/15 a las 14:07, Jan Schermer escribió:
>> Yes, but that’s a separate issue :-)
>> Some drives are just slow (100 IOPS) for synchronous writes with no other load.
>> The drives I’m testing have ~8K IOPS when not under load - having them drop to 10 IOPS is a huge problem. If it’s indeed a CFQ problem (as I suspect) then no matter what drive you have you will have problems.
>>
>> Jan
>>
>>> On 23 Jun 2015, at 14:03, Dan van der Ster <dan@xxxxxxxxxxxxxx> wrote:
>>>
>>> Oh sorry, I had missed that. Indeed that is surprising. Did you read
>>> the recent thread ("SSD IO performance") discussing the relevance of
>>> O_DSYNC performance for the journal?
>>>
>>> Cheers, Dan
>>>
>>> On Tue, Jun 23, 2015 at 1:54 PM, Jan Schermer <jan@xxxxxxxxxxx> wrote:
>>>> I only use SSDs, which is why I’m so surprised at the CFQ behaviour - the drive can sustain tens of thousand of reads per second, thousands of writes - yet saturating it with reads drops the writes to 10 IOPS - that’s mind boggling to me.
>>>>
>>>> Jan
>>>>
>>>>> On 23 Jun 2015, at 13:43, Dan van der Ster <dan@xxxxxxxxxxxxxx> wrote:
>>>>>
>>>>> On Tue, Jun 23, 2015 at 1:37 PM, Jan Schermer <jan@xxxxxxxxxxx> wrote:
>>>>>> Yes, I use the same drive
>>>>>>
>>>>>> one partition for journal
>>>>>> other for xfs with filestore
>>>>>>
>>>>>> I am seeing slow requests when backfills are occuring - backfills hit the filestore but slow requests are (most probably) writes going to the journal - 10 IOPS is just to few for anything.
>>>>>>
>>>>>>
>>>>>> My Ceph version is dumpling - that explains the integers.
>>>>>> So it’s possible it doesn’t work at all?
>>>>> I thought that bug was fixed. You can check if it worked by using
>>>>> "iotop -b -n1" and looking for threads with the idle priority.
>>>>>
>>>>>> Bad news about the backfills no being in the disk thread, I might have to use deadline after all.
>>>>> If your experience follows the same paths of most users, eventually
>>>>> deep scrubs will cause latency issues and you'll switch back to cfq
>>>>> plus ionicing the disk thread.
>>>>>
>>>>> Are you using Ceph RBD or object storage? If RBD, eventually you'll
>>>>> find that you need to put the journals on an SSD.
>>>>>
>>>>> Cheers, Dan
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>
>
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