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author | Kirk McKusick <mckusick@FreeBSD.org> | 2022-08-15 20:44:49 +0000 |
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committer | Kirk McKusick <mckusick@FreeBSD.org> | 2022-08-15 20:45:48 +0000 |
commit | 82493ff7007abb5a2d059c74748d2907dafc5395 (patch) | |
tree | 8d8201c6411c1ee7adc13e09dcbfecac5b073bfd | |
parent | 88a782fc8470a102d6fe51fe993173f67e77ea6d (diff) | |
download | src-82493ff7007abb5a2d059c74748d2907dafc5395.tar.gz src-82493ff7007abb5a2d059c74748d2907dafc5395.zip |
Add a description of soft updates journaling to tunefs(8).
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
-rw-r--r-- | sbin/tunefs/tunefs.8 | 40 |
1 files changed, 40 insertions, 0 deletions
diff --git a/sbin/tunefs/tunefs.8 b/sbin/tunefs/tunefs.8 index 065844a831cd..5ad0210694d4 100644 --- a/sbin/tunefs/tunefs.8 +++ b/sbin/tunefs/tunefs.8 @@ -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 |