Re: Read-only ReiserFS support for FreeBSD 5.x

In the last episode (Oct 19), Ryan Sommers said:
> It is my understanding that SoftUpdates offers the same guarantees
> that journaling does. It ensures file-system consistancy even in the
> event of system failure. I believe the only reason fsck is still run
> is to clean up the few cases that SoftUpdates can't detect, which is
> just block bitmaps and inode link counts. Neither of these are
> critical in that they can both be performed by a background fsck on a
> snapshot of a live filesystem.

Softupdates guarantees only that the filesystem will be in a consistent
state.  That state may be missing directories created up to
kern.dirdelay seconds ago, files created up to kern.filedelay seconds
ago, and metadata changed up to kern.metadelay seconds ago.  I
recommend cranking down all three values to under 10 seconds if you are
worried about losing recently-modified files after a crash.

Journalled filesystems may guarantee that the filesystem will be
consistent to a particular point in time, so if your editor updates
(for example) rc.conf by creating a new copy named rc.conf.new,
deleting rc.conf, then renaming the copy to rc.conf, you are guaranteed
that either rc.conf or rc.conf.new will be in the filessytem after a
crash.  With softupdates, you may lose both files.  I don't know if
current journalled fses actually journal all writes like this (as
opposed to just journalling what the OS decides to flush out of its
cache), but they are much less likely to lose files the way softupdates
can.

-- 
	Dan Nelson
	dnelson_at_allantgroup.com