[Bug 261944] FFS/UFS: explain soft update journaling (SUJ) in documentation and in tunefs(8)

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261944

Kirk McKusick <mckusick@FreeBSD.org> changed:

           What    |Removed                     |Added
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             Status|New                         |In Progress

--- Comment #9 from Kirk McKusick <mckusick@FreeBSD.org> ---
(In reply to Graham Perrin from comment #8)
Here is an expanded explanation of journaled soft updates:

I will leave it to you / others to figure out where it should go. My take is
that all but the last paragraph should at least be added to tunefs(8).

=-=-=

Soft updates can add journaling that reduces the time spent by fsck_ffs(8)
cleaning up a filesystem after a crash to a few seconds from minutes to hours.
It is possible to run fsck_ffs(8) in background on a live filesystem though the
filesystem performance will be about half of normal during the time that the
background fsck_ffs(8) is running.

Without journaling, the time to recover after a crash is a function of the
number of files in the filesystem and the size of the filesystem.  With
journaling, the time to recover after a crash is a function of the amount of
activity in the filesystem in the minute before the crash.  Journaled recovery
time is usually only a few seconds and never exceeds a minute.

The drawback to using journaling is that the writes to its log adds an extra
write load to the media containing the filesystem. Thus a write-intensive
workload will have reduced throughput on a filesystem running with journaling.

Like all journaling filesystems, the journal recovery will only fix issues
known to the journal. Specifically if a media error occurs, the journal will
not know about it and hence will not fix it. Thus when using journaling, it is
still necessary to run a full fsck every few months or after a filesystem panic
to check for and fix any errors brought on by media failure. A full fsck can be
done by running a background fsck on a live filesystem or by running with the
-f flag on an unmounted filesystem. Running a full fsck on a UFS filesystem is
the equivalent of running a scrub on a ZFS filesystem.

Presently it is not possible to take a snapshot on a UFS filesystem running
with journaled soft updates. Thus it is not possible to reliably dump mounted
filesystems or to run background fsck on them.  The FreeBSD Foundation has
sponsored me to make snapshots possible on journaled filesystems. I anticipate
having that project completed by July 2023.

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