HAMMER update 09-June-2008

[Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>]

    With the 53D commit HAMMER has stabilized again.  I will again
    recommend that people testing HAMMER update and newfs_hammer your
    filesystems.  53D located and corrected a data overwrite bug and
    53C corrected a freemap bug that could cause pruning/reblocking panics.

    I am continuing to work on various performance issues.  In particular,
    when I run blogbench I can make the system become extremely inefficient
    and writes initiated by the flusher, which are not subject to kernel
    restrictions, can 'take over' the system and cause everything else
    trying to write to a file to block for very long periods of time.

    I am considering making another change to the on-media format to
    increase the size of the B-Tree node from 16 elements to 64 elements.
    B-Tree operations appear to be HAMMER's only major hangup right now.
    Blogbench has shown that B-Tree updates can wind up being extremely
    disk-inefficient.  I have made a final decision on the matter yet,
    I need to play with the flusher's B-Tree updates for a few days first.

						-Matt