freebsd-src/sbin/tunefs/tunefs.8

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.Dd November 17, 2023
.Dt TUNEFS 8
.Os
.Sh NAME
.Nm tunefs
.Nd tune up an existing UFS file system
.Sh SYNOPSIS
.Nm
.Op Fl A
.Op Fl a Cm enable | disable
.Op Fl e Ar maxbpg
.Op Fl f Ar avgfilesize
.Op Fl j Cm enable | disable
.Op Fl J Cm enable | disable
.Op Fl k Ar held-for-metadata-blocks
.Op Fl L Ar volname
.Op Fl l Cm enable | disable
.Op Fl m Ar minfree
.Op Fl N Cm enable | disable
.Op Fl n Cm enable | disable
.Op Fl o Cm space | time
.Op Fl p
.Op Fl s Ar avgfpdir
.Op Fl S Ar size
.Op Fl t Cm enable | disable
.Ar special | filesystem
.Sh DESCRIPTION
The
.Nm
utility is designed to change the dynamic parameters of a UFS file system
which affect the layout policies.
The
.Nm
utility cannot be run on an active file system.
To change an active file system,
it must be downgraded to read-only or unmounted.
.Pp
The parameters which are to be changed are indicated by the flags
given below:
.Bl -tag -width indent
.It Fl A
The file system has several backups of the super-block.
Specifying
this option will cause all backups to be modified as well as the
primary super-block.
This is potentially dangerous - use with caution.
.It Fl a Cm enable | disable
Turn on/off the administrative POSIX.1e ACL enable flag.
.It Fl e Ar maxbpg
Indicate the maximum number of blocks any single file can
allocate out of a cylinder group before it is forced to begin
allocating blocks from another cylinder group.
Typically this value is set to about one quarter of the total blocks
in a cylinder group.
The intent is to prevent any single file from using up all the
blocks in a single cylinder group,
thus degrading access times for all files subsequently allocated
in that cylinder group.
The effect of this limit is to cause big files to do long seeks
more frequently than if they were allowed to allocate all the blocks
in a cylinder group before seeking elsewhere.
For file systems with exclusively large files,
this parameter should be set higher.
.It Fl f Ar avgfilesize
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.
.It Fl J Cm enable | disable
Turn on/off gjournal flag.
.It Fl k Ar held-for-metadata-blocks
Set the amount of space to be held for metadata blocks.
When set, the file system preference routines will try to save
the specified amount of space immediately following the inode blocks
in each cylinder group for use by metadata blocks.
Clustering the metadata blocks speeds up random file access
and decreases the running time of
.Xr fsck 8 .
While this option can be set at any time,
it is most effective if set before any data is loaded into the file system.
By default
.Xr newfs 8
sets it to half of the space reserved to minfree.
.It Fl L Ar volname
Add/modify an optional file system volume label.
Legal characters are alphanumerics, dashes, and underscores.
.It Fl l Cm enable | disable
Turn on/off MAC multilabel flag.
.It Fl m Ar minfree
Specify the percentage of space held back
from normal users; the minimum free space threshold.
The default value used is 8%.
Note that lowering the threshold can adversely affect performance:
.Bl -bullet
.It
Settings of 5% and less force space optimization to
always be used which will greatly increase the overhead for file
writes.
.It
The file system's ability to avoid fragmentation will be reduced
when the total free space, including the reserve, drops below 15%.
As free space approaches zero, throughput can degrade by up to a
factor of three over the performance obtained at a 10% threshold.
.El
.Pp
If the value is raised above the current usage level,
users will be unable to allocate files until enough files have
been deleted to get under the higher threshold.
.It Fl N Cm enable | disable
Turn on/off the administrative NFSv4 ACL enable flag.
.It Fl n Cm enable | disable
Turn on/off soft updates.
.It Fl o Cm space | time
The file system can either try to minimize the time spent
allocating blocks, or it can attempt to minimize the space
fragmentation on the disk.
Optimization for space has much
higher overhead for file writes.
The kernel normally changes the preference automatically as
the percent fragmentation changes on the file system.
.It Fl p
Show a summary of what the current tunable settings
are on the selected file system.
More detailed information can be
obtained from the
.Xr dumpfs 8
utility.
.It Fl s Ar avgfpdir
Specify the expected number of files per directory.
.It Fl S Ar size
Specify the softdep journal size in bytes.
The minimum is 4M.
.It Fl t Cm enable | disable
Turn on/off the TRIM enable flag.
If enabled, and if the underlying device supports the BIO_DELETE
command, the file system will send a delete request to the underlying
device for each freed block.
The trim enable flag is typically set when the underlying device
uses flash-memory as the device can use the delete command to
pre-zero or at least avoid copying blocks that have been deleted.
.Pp
Note that this does not trim blocks that are already free.
See the
.Xr fsck_ffs 8
.Fl E
flag.
.El
.Pp
At least one of these flags is required.
.Sh FILES
.Bl -tag -width ".Pa /etc/fstab"
.It Pa /etc/fstab
read this to determine the device file for a
specified mount point.
.El
.Sh SEE ALSO
.Xr ffs 4 ,
.Xr fs 5 ,
.Xr tuning 7 ,
.Xr dumpfs 8 ,
.Xr gjournal 8 ,
.Xr growfs 8 ,
.Xr newfs 8
.Rs
.%A M. McKusick
.%A W. Joy
.%A S. Leffler
.%A R. Fabry
.%T "A Fast File System for UNIX"
.%J "ACM Transactions on Computer Systems 2"
.%N 3
.%P pp 181-197
.%D August 1984
.%O "(reprinted in the BSD System Manager's Manual, SMM:5)"
.Re
.Sh HISTORY
The
.Nm
utility appeared in
.Bx 4.2 .
.Sh BUGS
This utility does not work on active file systems.
To change the root file system, the system must be rebooted
after the file system is tuned.
.\" Take this out and a Unix Daemon will dog your steps from now until
.\" the time_t's wrap around.
.Pp
You can tune a file system, but you cannot tune a fish.