Fwd: Resize HAMMER filesystem

Matthew Dillon dillon at apollo.backplane.com
Fri May 3 10:02:54 PDT 2013

    HAMMER1 has a number of deficiencies in this area.  It is possible
    to add additional block devices to a HAMMER1 filesystem but it's never
    been well tested and I don't recommend it.

    HAMMER1's best feature is mirror-copy and mirror-stream.  There are
    deficiencies here as well, particularly the issue with the root 
    filesystem not being a PFS.  Mirroring slaves can be upgraded to masters
    but they cannot be downgraded again, so their best use is to track
    near real-time backups.

    Snapshots also work extremely well and the fine-grained history works
    quite well too.

    The multi-volume feature doesn't work as well as we would like, and
    HAMMER1 doesn't have any redundancy so if you lose a HD that filesystem
    is basically dead (would have to be rebuilt from one of the mirror
    slaves being used as a backup).

    HAMMER2 will solve a lot of these problems.  HAMMER2 has a 'super-root'
    directory level above the root mount so mount points are basically just
    PFS's, including the root mount.  Snapshots are writable, and there will
    be a copies mechanism that treats multiple volumes as separate logical
    filesystems (i.e. real redundancy), and will also be able to take on the
    duty of adding/removing storage from a live filesystem.

    HAMMER2 is not even remotely close to being production ready yet though.

    At this point in time I am only doing minor maintainance work on HAMMER1.
    All my efforts are going into HAMMER2.


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