HAMMER is going to be a little unstable as I commit the crash

    recovery code.  I'm about half way through it.  Meta-data updates

    to the disk media have now been separated out.  I have a few things

    left to do before crash recovery will actually work:
    * I have to flush the undo buffers out before the meta-data buffers

    * Then I have to flush the volume header so mount can see the updated

      undo info.

    * Then I have to flush out the meta-data buffers that the UNDO

      info refers to.

    * And, finally, the mount code must scan the UNDO buffers and perform

      any required UNDOs.
    The idea being that if a crash occurs at any point in the above

    sequence, HAMMER will be able to run the UNDOs to undo any partially

    written meta-data.  HAMMER would be able to do this at mount-time and

    it would probably take less then a second, so basically this gives us

    our instant crash-recovery feature.
    One interesting outcome of the separation work I just committed is

    that the frontend VOPs are *massively* disconnected from backend disk

    I/O now.  In coming weeks I hope to take advantage of this separation

    to remove the remaining stalls and significantly improve HAMMER's

    performance.
						-Matt