On Tue, 22 May 2012 15:59:50 +0300 Alexander Lyakas <alex.bolshoy@xxxxxxxxx> wrote: > Hi Neil, > can you pls give some details on that commit. > > As far as I understand, this change attempts to protect from > split-brain, most typical to RAID1 (but also, e.g., to 4-drive RAID6) > , where part of a mirrored set was assembled independently. The code > first selects "most_recent" based on event count (as usual). Then it > applies the map check to all those devices that are not "most_recent", > and might kick them out, if it detects split-brain. > However, when there is such split-brain, and parts of mirrored sets > are assembled independently, the highest event count does not really > tell us which part of the mirrored set is "more up-to-date". This is > because event count is not tied to any hard clock or something like > that. So there is really no way to tell what part of the mirrored set > will be picked up here (WRT to user activity on the separate mirrored > sets). In a split brain situation *neither* side is "more up-to-date". They are both simply "differently up-to-date". A wall-clock based event count would not change this fact. > > What I am trying to say, I guess: don't you think that in such case, > it would be better to warn the user and abort, and not pick (more or > less) arbitrary part of the set? Or, in other words:) might you > reconsider looking at some ideas for split-brain protection I pitched > some time ago?:)) This is a policy question and so I am happy for an extension to the new "policy" mechanism in mdadm to allow finer control for managing it. I'm fairy sure that I think the default should be the current behaviour. If you are assembling the arrays with "-I" it not really possible to reject the first half of the brain that is found, so I don't think we should when assembling with "-A". I'm afraid I don't particularly remember the ideas you pitched before. Feel free to pitch them again -- and repeat every few weeks until you get an answer :-) NeilBrown
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