[ceph-users] Slow requests in cache tier with rep_size 2

Eugen Block eblock at nde.ag
Wed Nov 1 08:23:48 PDT 2017


Hi David,

> What is your min_size in the cache pool?  If your min_size is 2, then the
> cluster would block requests to that pool due to it having too few copies
> available.

this is a little embarassing, but it seems it was the min_size indeed.  
I had changed this setting a couple of weeks ago, but as I found out,  
a colleague had to rebuild the cache pool. And since we have defined  
the default min_size of 2 for new pools in our ceph.conf my changes  
were overwritten, of course.

A little test where I stopped one of the two SSDs after changing  
min_size back to 1 was successful, I don't see any hints for slow  
requests, also no co-worker complained about hanging VMs :-)

Thanks for pointing that out (again), I'll try to convince my employer  
to get a third SSD ;-)

Regards,
Eugen


Zitat von David Turner <drakonstein at gmail.com>:

> PPS - or min_size 1 in production
>
> On Wed, Nov 1, 2017 at 10:08 AM David Turner <drakonstein at gmail.com> wrote:
>
>> What is your min_size in the cache pool?  If your min_size is 2, then the
>> cluster would block requests to that pool due to it having too few copies
>> available.
>>
>> PS - Please don't consider using rep_size 2 in production.
>>
>> On Wed, Nov 1, 2017 at 5:14 AM Eugen Block <eblock at nde.ag> wrote:
>>
>>> Hi experts,
>>>
>>> we have upgraded our cluster to Luminous successfully, no big issues
>>> so far. We are also testing cache tier with only 2 SSDs (we know it's
>>> not recommended), and there's one issue to be resolved:
>>>
>>> Evertime we have to restart the cache OSDs we get slow requests with
>>> impacts on our client VMs. We never restart both SSDs at the same
>>> time, of course, so we are wondering why the slow requests? What
>>> exactly is happening with a replication size of 2? We assumed that if
>>> one SSD is restarted the data is still available on the other disc, we
>>> didn't expect any problems. But this occurs everytime we have to
>>> restart them. Can anyone explain what is going on there? We could use
>>> your expertise on this :-)
>>>
>>> Best regards,
>>> Eugen
>>>
>>> --
>>> Eugen Block                             voice   : +49-40-559 51 75
>>> <+49%2040%205595175>
>>> NDE Netzdesign und -entwicklung AG      fax     : +49-40-559 51 77
>>> <+49%2040%205595177>
>>> Postfach 61 03 15
>>> D-22423 Hamburg                         e-mail  : eblock at nde.ag
>>>
>>>          Vorsitzende des Aufsichtsrates: Angelika Mozdzen
>>>            Sitz und Registergericht: Hamburg, HRB 90934
>>>                    Vorstand: Jens-U. Mozdzen
>>>                     USt-IdNr. DE 814 013 983
>>>
>>> _______________________________________________
>>> ceph-users mailing list
>>> ceph-users at lists.ceph.com
>>> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
>>>
>>



-- 
Eugen Block                             voice   : +49-40-559 51 75
NDE Netzdesign und -entwicklung AG      fax     : +49-40-559 51 77
Postfach 61 03 15
D-22423 Hamburg                         e-mail  : eblock at nde.ag

         Vorsitzende des Aufsichtsrates: Angelika Mozdzen
           Sitz und Registergericht: Hamburg, HRB 90934
                   Vorstand: Jens-U. Mozdzen
                    USt-IdNr. DE 814 013 983



More information about the ceph-users mailing list