On Wed, Jun 7, 2017 at 12:59 PM Jonas Jaszkowic < jonasjaszko...@googlemail.com> wrote:
> If you set min_size 2 before taking the OSDs down, that does seem odd. > > > I think I don’t get the exact concept of min_size in the crush Crush > ruleset. The documentation ( > http://docs.ceph.com/docs/master/rados/operations/crush-map/) states: > > min_size > Description: If a pool makes fewer replicas than this number, CRUSH > will NOT select this rule. > Type: Integer > Purpose: A component of the rule mask. > Required: Yes > Default: 1 > > Assuming that I want my scenario to work (5 OSDs, 2+3 EC Pool, 3 OSDs > down, still reading my data), how do > I have to configure my pool exactly to work? Or is this simply not > possible at this point? > The CRUSH rule min_size is a completely different thing from the pool min_size. If you set the pool min_size to 2 I *think* it will do what you expect. But in general running with min_size == k is not a wise way to run the cluster as you don't have any redundancy in the case of losses. :) > I just want to be sure that I have no errors in my configuration. > > Yeah, we just don't have a way of serving reads without serving writes at > the moment. It's a limit of the architecture. > > > Thank you, this is good to know, particularly because I didn’t find > anything about it on the documentation. > > - Jonas > > > Am 07.06.2017 um 21:40 schrieb Gregory Farnum <gfar...@redhat.com>: > > > > On Wed, Jun 7, 2017 at 12:30 PM Jonas Jaszkowic < > jonasjaszko...@googlemail.com> wrote: > >> >> Am 07.06.2017 um 20:29 schrieb Gregory Farnum <gfar...@redhat.com>: >> >> We prevent PGs from going active (and serving writes or reads) when they >> have less than "min_size" OSDs participating. This is generally set so that >> we have enough redundancy to recover from at least one OSD failing. >> >> >> Do you mean the min_size value from the crush rule? I set min_size = 2, >> so a 2+3 EC pool with 3 killed OSDs still has the minimum amount of 2 OSDs >> and should be able >> to fully recover data, right? >> > > If you set min_size 2 before taking the OSDs down, that does seem odd. > > >> >> In your case, you have 2 OSDs and the failure of either one of them >> results in the loss of all written data. So we don't let you go active as >> it's not safe. >> >> >> I get that it makes no sense to serve *writes* at this point because we >> cannot provide the desired redundancy, but how is preventing me from going >> active more safe than just serving *reads*? I think what bugs me is that >> by definition of the used erasure code, we should be able to loose 3 OSDs >> and still get our data back - which is not the case in this scenario >> because our cluster refuses to go active. >> > > Yeah, we just don't have a way of serving reads without serving writes at > the moment. It's a limit of the architecture. > > -Greg > PS: please keep this on the list. It spreads the information and archives > it for future reference by others. :) > > >
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