On 08/15/2014 12:23 PM, Loic Dachary wrote: > Hi Erik, > > On 15/08/2014 11:54, Erik Logtenberg wrote: >> Hi, >> >> With EC pools in Ceph you are free to choose any K and M parameters you >> like. The documentation explains what K and M do, so far so good. >> >> Now, there are certain combinations of K and M that appear to have more >> or less the same result. Do any of these combinations have pro's and >> con's that I should consider and/or are there best practices for >> choosing the right K/M-parameters? >> Loic might have a better anwser, but I think that the more segments (K) you have, the heavier recovery. You have to contact more OSDs to reconstruct the whole object so that involves more disks doing seeks. I heard sombody from Fujitsu say that he thought 8/3 was best for most situations. That wasn't with Ceph though, but with a different system which implemented Erasure Coding. >> For instance, if I choose K = 3 and M = 2, then pg's in this pool will >> use 5 OSD's and sustain the loss of 2 OSD's. There is 40% overhead in >> this configuration. >> >> Now, if I were to choose K = 6 and M = 4, I would end up with pg's that >> use 10 OSD's and sustain the loss of 4 OSD's, which is statistically not >> so much different from the first configuration. Also there is the same >> 40% overhead. > > Although I don't have numbers in mind, I think the odds of loosing two OSD simultaneously are a lot smaller than the odds of loosing four OSD simultaneously. Or am I misunderstanding you when you write "statistically not so much different from the first configuration" ? > Loosing two smaller then loosing four? Is that correct or did you mean it the other way around? I'd say that loosing four OSDs simultaneously is less likely to happen then two simultaneously. > Cheers > >> One rather obvious difference between the two configurations is that the >> latter requires a cluster with at least 10 OSD's to make sense. But >> let's say we have such a cluster, which of the two configurations would >> be recommended, and why? >> >> Thanks, >> >> Erik. >> _______________________________________________ >> ceph-users mailing list >> ceph-users at lists.ceph.com >> http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >> > > > > _______________________________________________ > ceph-users mailing list > ceph-users at lists.ceph.com > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com > -- Wido den Hollander 42on B.V. Phone: +31 (0)20 700 9902 Skype: contact42on
 |