Unable to mount hammer file system Undo failed

[Matthew Dillon]

    People who use HAMMER also tend to backup their filesystems using
    the streaming mirroring feature.  You need a backup anyway, regardless.
    HAMMER makes it easy, and this is the recommended method for dealing
    with media faults on HDDs not backed by hardware RAID (and even if
    they are).  You need to backup your data anyway, after all, regardless
    of the filesystem (even ZFS's 'copies' feature has its limits due to
    the fact that the copies are all being managed from the same machine).

    FreeBSD's background fsck and mounting without an fsck (depending on
    softupdates) has NEVER been well vetted to ensure that it works in
    all situations.  There have been lots of complaints about related
    failures over the years, mostly blamed on failed writes to disks or
    people not having UPS's (since UFS was never designed to work with
    a disk synchronization command, crashes from e.g. power failures could
    seriously corrupt disks above and beyond lost sectors).  They can
    claim it works better now, but I would never trust it.  Background fsck
    itself can render a server unusable due to lost performance.

    HAMMER has a 'hammer recover' command meant to be used when all else
    fails.  It can be used directly with the bad/corrupted disk as the source
    and a new disk as the destination.  It scans the disk, yes.  A full
    fsck on a very large (2TB+) filled filesystem is almost as bad when it
    starts having to seek around.

    I have had numerous failed disks over the years and have never had to
    actually use the recover command.  I always initialize a replacement
    from one of the several live backups I keep.

    HAMMER2 will have some more interesting features that flesh out the
    live backup mechanic a bit better, making it possible to e.g. initialize
    a replacement disk locally and leave the filesystem live using a remotely
    served backup as the replacement is reloaded from the backup.  But it
    isn't possible with HAMMER1, sorry.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>