Le 22/07/2015 21:17, Lincoln Bryant a écrit : > Hi Hadi, > > AFAIK, you can’t safely mount RBD as R/W on multiple machines. You > could re-export the RBD as NFS, but that’ll introduce a bottleneck and > probably tank your performance gains over CephFS. > > For what it’s worth, some of our RBDs are mapped to multiple machines, > mounted read-write on one and read-only on the others. We haven’t seen > any strange effects from that, but I seem to recall it being ill advised.
Yes it is, for several reasons. Here are two at the top of my head. Some (many/most/all?) filesystems update on-disk data when they are mounted even if the mount is read-only. If you map your RBD devices read-only before mounting the filesystem itself read-only you should be safe from corruption occurring at mount time though. The system with read-write access will keep its in-memory data in sync with the on-disk data. The others with read-only access will not as they won't be aware of the writes done, this means they will eventually get incoherent data and will generate fs access errors with various levels of errors from the benign read error to potentially full kernel crash with whole filesystem freeze in-between. Don't do that unless you : - carefully setup your rbd mappings read-only everywhere but the system doing the writes, - can withstand a (simultaneous) system crash on all the systems mounting the rbd mappings read-only. Lionel
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