On Tue, Apr 10, 2018 at 5:54 AM, John Spray <jspray@xxxxxxxxxx> wrote: > On Tue, Apr 10, 2018 at 1:44 PM, Yan, Zheng <ukernel@xxxxxxxxx> wrote: >> Hello >> >> To simplify snapshot handling in multiple active mds setup, we changed >> format of snaprealm in mimic dev. >> https://github.com/ceph/ceph/pull/16779. >> >> The new version mds can handle old format snaprealm in single active >> setup. It also can convert old format snaprealm to the new format when >> snaprealm is modified. The problem is that new version mds can not >> properly handle old format snaprealm in multiple active setup. It may >> crash when it encounter old format snaprealm. For existing filesystem >> with snapshots, upgrading mds to mimic seems to be no problem at first >> glance. But if user later enables multiple active mds, mds may >> crashes continuously. No easy way to switch back to single acitve mds. >> >> I don't have clear idea how to handle this situation. I can think of a >> few options. >> >> 1. Forbid multiple active before all old snapshots are deleted or >> before all snaprealms are converted to new format. Format conversion >> requires traversing while whole filesystem tree. Not easy to >> implement. > > This has been a general problem with metadata format changes: we can > never know if all the metadata in a filesystem has been brought up to > a particular version. Scrubbing (where scrub does the updates) should > be the answer, but we don't have the mechanism for recording/ensuring > the scrub has really happened. > > Maybe we need the MDS to be able to report a complete whole-filesystem > scrub to the monitor, and record a field like > "latest_scrubbed_version" in FSMap, so that we can be sure that all > the filesystem metadata has been brought up to a certain version > before enabling certain features? So we'd then have a > "latest_scrubbed_version >= mimic" test before enabling multiple > active daemons. > > For this particular situation, we'd also need to protect against > people who had enabled multimds and snapshots experimentally, with an > MDS startup check like: > ((ever_allowed_features & CEPH_MDSMAP_ALLOW_SNAPS) && > (allows_multimds() || in.size() >1)) && latest_scrubbed_version < > mimic This sounds like the right approach to me. The mons should also be capable of performing the same test and raising a health error that pre-Mimic MDSs must be started and the number of actives be reduced to 1. -- Patrick Donnelly _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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