On Tue, May 8, 2018 at 12:07 PM, Dan van der Ster <dan@xxxxxxxxxxxxxx> wrote: > On Tue, May 8, 2018 at 7:35 PM, Vasu Kulkarni <vakulkar@xxxxxxxxxx> wrote: >> On Mon, May 7, 2018 at 2:26 PM, Maciej Puzio <mkp37215@xxxxxxxxx> wrote: >>> I am an admin in a research lab looking for a cluster storage >>> solution, and a newbie to ceph. I have setup a mini toy cluster on >>> some VMs, to familiarize myself with ceph and to test failure >>> scenarios. I am using ceph 12.2.4 on Ubuntu 18.04. I created 5 OSDs >>> (one OSD per VM), an erasure-coded pool for data (k=3, m=2), a >>> replicated pool for metadata, and CephFS on top of them, using default >>> settings wherever possible. I mounted the filesystem on another >>> machine and verified that it worked. >>> >>> I then killed two OSD VMs with an expectation that the data pool will >>> still be available, even if in a degraded state, but I found that this >>> was not the case, and that the pool became inaccessible for reading >>> and writing. I listed PGs (ceph pg ls) and found the majority of PGs >>> in an incomplete state. I then found that the pool had size=5 and >>> min_size=4. Where did the value 4 come from, I do not know. >>> >>> This is what I found in the ceph documentation in relation to min_size >>> and resiliency of erasure-coded pools: >>> >>> 1. According to >>> http://docs.ceph.com/docs/luminous/rados/operations/pools/ the values >>> size and min_size are for replicated pools only. >>> 2. According to the same document, for erasure-coded pools the number >>> of OSDs that are allowed to fail without losing data equals the number >>> of coding chunks (m=2 in my case). Of course data loss is not the same >>> thing as lack of access, but why these two things happen at different >>> redundancy levels, by default? >>> 3. The same document states that that no object in the data pool will >>> receive I/O with fewer than min_size replicas. This refers to >>> replicas, and taken together with #1, appear not to apply to >>> erasured-coded pools. But in fact it does, and the default min_size != >>> k causes a surprising behavior. >>> 4. According to >>> http://docs.ceph.com/docs/master/rados/operations/pg-states/ , >>> reducing min_size may allow recovery of an erasure-coded pool. This >>> advice was deemed unhelpful and removed from documentation (commit >>> 9549943761d1cdc16d72e2b604bf1f89d12b5e13), but then re-added (commit >>> ac6123d7a6d27775eec0a152c00e0ff75b36bd60). I guess I am not the only >>> one confused. >> >> >> you bring up good inconsistency that needs to be addressed, afaik,only >> m value is important >> for ec pools, i am not sure if the *replicated* metadata pool is >> somehow causing min_size >> variance in your experiment to work. when we create replicated pool it >> has option for min size >> and for ec pool it is the m value. > > See https://github.com/ceph/ceph/pull/8008 for the reason why min_size > defaults to k+1 on ec pools. So this looks like its happening by default per ec pool, unless user is changing the pool min_size. probably this should be left unchanged and we could document it? It is bit confusing with coding chunks. > > Cheers, Dan _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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