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Add a description of soft updates journaling to tunefs(8).

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Add a descrition to the tunefs(8) -j (journal enablement) flag
that explains what soft updates journaling does, the tradeoffs
to using it, and the limitations that it imposes.

Requested by: Graham Perrin
PR:           261944
Sponsored by: The FreeBSD Foundation
This commit is contained in:
Kirk McKusick 2022-08-15 13:44:49 -07:00
parent 88a782fc84
commit 82493ff700
1 changed files with 40 additions and 0 deletions
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sbin/tunefs/tunefs.8
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@ -95,6 +95,46 @@ this parameter should be set higher.
Specify the expected average file size.
.It Fl j Cm enable | disable
Turn on/off soft updates journaling.
.Pp
Enabling journaling reduces the time spent by
.Xr fsck_ffs 8
cleaning up a filesystem after a crash to a few seconds from minutes to hours.
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.
.Pp
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.
.Pp
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
.Fl f
flag on an unmounted filesystem.
When running
.Xr fsck_ffs 8
in background on a live filesystem the filesystem performance
will be about half of normal during the time that the background
.Xr fsck_ffs 8
is running.
Running a full fsck on a UFS filesystem is the equivalent of
running a scrub on a ZFS filesystem.
.Pp
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 filesystems enabled for journaling.
.It Fl J Cm enable | disable
Turn on/off gjournal flag.
.It Fl k Ar held-for-metadata-blocks