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Thu, 15 May 2008

POHMELFS distributed plans.

After healthy discussion started after my announcement of the second POHMELFS release, its time to highlight main ideas settled in the thread.

First, POHMELFS will be moved into parallel distributed filesystems, but still being very good as network filesystem. In particular, that will include ability to read data from one of the connected server (not particulary from currently active, how its done right now), writing will happen to all connected servers simultaneously (and transaction will be committed after all servers returned completion acknowledge).

Protocol will be extended to support dynamic addtion and removal of the servers to/from currently connected group. Probably there will be some kind of a status messages for servers (i.e. going offline, do not send me data, or I'm becoming slow, do not read from me and so on). It will be done in addition to cache coherency messages (I'm yet to implement, but because of other tasks, this was a bit postponed, probably to weekend), which will include two types of requests: page invalidation and inode update (that will also mean that POHMELFS will start supporting attributes (maybe even extended), right now it doesn't :). Such cache coherency protocol should scale better than classical MOSI (and its derivatives) and particulary better than pNFS spec proides (leases to operations for some servers), since it is still possible to work in parallel with the same file, especially without any overhead of data processing does not cross different client boundaries, but it has to be tested in practice.

POHMELFS server will be extended to support distributed facilities. Very likely it will be some kind of PAXOS algorithm, although probably in its very limited mode for the beginning. So far it will be really simple, so that I could touch all its corner cases and found optimal development strategy.

All client extensions are rather not that complex, although not always trivial, so that should not take too much time, so probably you will get something interesting soon.
Server extensions will be a bit slower, since I will start essentially from the distributed system ground and gradually move upstairs.

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