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DragonFly kernel List (threaded) for 2011-07
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Re: HAMMER2 design in progress - 1-2 year time frame

From: Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>
Date: Sat, 2 Jul 2011 23:40:53 -0700 (PDT)

    Well, the problem with hardlinks in HAMMER2 is a bit different than
    tagging files and directories with additional meta-data.

    Meta-data is actually very easy to do from a cache coherency standpoint,
    but difficult to reliably copy and archive (particularly across
    filesystem types).

    Hardlinks create multiple synchronization/coherency paths from each link
    (to the same file) all the way back to the root.  In a clustered
    filesystem with fine-grained cache coherency on the DIRECTORY TOPOLOGY,
    verses just the file inode, this can wind up being O(n log n) in overhead,
    approximately, for each system call operating on the file (where (n) is
    the number of hard links).  Performance suffers badly and the cache
    coherency algorithms also wind up being far more complex... too complex
    to be desireable, really.

    I'll continue to think about how at least a small number of hardlinks
    could be implemented without turning already complex cluster algorithms
    into a disaster zone.  So far I'm drawing a blank.


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