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

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

PR:           261944
Sponsored by: The FreeBSD Foundation
This commit is contained in:
Kirk McKusick 2022-10-21 10:56:20 -07:00
parent 04f6b9cb18
commit 0929a153fc
1 changed files with 41 additions and 1 deletions
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42
sbin/newfs/newfs.8
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@ -28,7 +28,7 @@
.\" @(#)newfs.8 8.6 (Berkeley) 5/3/95
.\" $FreeBSD$
.\"
.Dd April 17, 2021
.Dd October 21, 2022
.Dt NEWFS 8
.Os
.Sh NAME
@ -169,6 +169,46 @@ This flag is implemented by running the
.Xr tunefs 8
utility found in the user's
.Dv $PATH .
.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 k Ar held-for-metadata-blocks
Set the amount of space to be held for metadata blocks in each cylinder group.
When set, the file system preference routines will try to save