Unable to mount hammer file system Undo failed
dillon at apollo.backplane.com
Thu Jul 19 21:02:54 PDT 2012
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.
<dillon at backplane.com>
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