Messages in this thread | | | Date | Mon, 21 Dec 2009 11:04:43 -0700 | From | Andreas Dilger <> | Subject | Re: [GIT PULL] Ceph distributed file system client for 2.6.33 |
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On 2009-12-21, at 09:42, Sage Weil wrote: > I think a compat/incompat flags mechanism during the > initial handshake might be appropriate to make changes easier going > forward.
Having compat/incompat flags for the network protocol, implemented correctly, is really critical for long term maintenance. For Lustre, we ended up using a single set of compatibility flags: - client sends full set of features that it understands - server replies with strict subset of flags that it also understands (i.e. client_features & server_supported_features) - if client doesn't have required support for a feature needed by the server, server refuses to allow client to mount - if server doesn't have feature required by client (e.g. understands only some older implementation no longer supported by client), client refuses to mount filesystem
We've been able to use this mechanism for the past 5 years to maintain protocol interoperability for Lustre, though we don't promise perpetual interoperability, only for about 3 years or so before users have to upgrade to a newer release. That allows us to drop support for ancient code instead of having to carry around baggage for every possible combination of old features.
Using simple version numbers for the protocol means you have to carry the baggage of every single previous version, and it isn't possible to have "experimental" features that are out in the wild, but eventually don't make sense to keep around forever.
Cheers, Andreas -- Andreas Dilger Sr. Staff Engineer, Lustre Group Sun Microsystems of Canada, Inc.
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