On Thu, Nov 8, 2018 at 3:41 AM Hector Martin <hec...@marcansoft.com> wrote:
> I'm experimenting with single-host Ceph use cases, where HA is not > important but data durability is. > > How does a Ceph cluster react to its (sole) mon being rolled back to an > earlier state? The idea here is that the mon storage may not be > redundant but would be (atomically, e.g. lvm snapshot and dump) backed > up, say, daily. If the cluster goes down and then is brought back up > with a mon backup that is several days to hours old, while the OSDs are > up to date, what are the potential consequences? > > Of course I expect maintenance operations to be affected (obviously any > OSDs added/removed would likely get confused). But what about regular > operation? Things like snapshots and snapshot ranges. Is this likely to > cause data loss, or would the OSDs and clients largely not be affected > as long as the cluster config has not changed? > > There's a way of rebuilding the monmap from OSD data: > > > http://docs.ceph.com/docs/mimic/rados/troubleshooting/troubleshooting-mon/#recovery-using-osds > > Would this be preferable to just restoring the mon from a backup? Yes, do that, don't try and back up your monitor. If you restore a monitor from backup then the monitor — your authoritative data source — will warp back in time on what the OSD peering intervals look like, which snapshots have been deleted and created, etc. It would be a huge disaster and probably every running daemon or client would have to pause IO until the monitor generated enough map epochs to "catch up" — and then the rest of the cluster would start applying those changes and nothing would work right. > What > about the MDS map? > Unlike the OSDMap, the MDSMap doesn't really keep track of any persistent data so it's much safer to rebuild or reset from scratch. -Greg > > -- > Hector Martin (hec...@marcansoft.com) > Public Key: https://mrcn.st/pub > _______________________________________________ > ceph-users mailing list > ceph-users@lists.ceph.com > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >
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