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

Eugen Block eblock at nde.ag
Wed Nov 1 02:14:05 PDT 2017

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 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

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