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Thu, 19 Jun 2008

CRFS got metadata cache coherency support.

Zach Brown has committed cache coherency support into CRFS repository.
Cache coherency protocol works by broadcasting special messages from server, and each client invalidates appropriate inodes (and dentries if needed) before sending back a reply.
POHMELFS uses a bit different mechanism: client does not send acks back to server, so all such messages are kind of advisory-only, but I did not yet complete (well, I did not even think about this problem this week) locking design, so it can change.

Main problem with sync cache coherency support is its absolute non-scalability. While number of sage cases might require such behaviour, I expect that if not major, but noticeble part of users do not want perfromance degradation as a price for posix-like coherency expectation. This approach is worse that write-through cache, since there is whole round-trip of the cache coherency request instead of just data sending during its writing. Single direction sending is faster than sending+waiting, so for me it is still a questionable approach.

I will think a lot of this problem later this week(end), so that solution would satisfy both high-perfomance and safety camps (although at some degree only I think).

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