.\"	$OpenBSD: tunefs.8,v 1.3 1996/08/02 11:23:34 deraadt Exp $
.\"	$NetBSD: tunefs.8,v 1.8 1995/03/18 15:01:29 cgd Exp $
.\"
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.\"     @(#)tunefs.8	8.2 (Berkeley) 12/11/93
.\"
.Dd December 11, 1993
.Dt TUNEFS 8
.Os BSD 4.2
.Sh NAME
.Nm tunefs
.Nd tune up an existing file system
.Sh SYNOPSIS
.Nm tunefs
.Op Fl A
.Op Fl a Ar maxcontig
.Op Fl d Ar rotdelay
.Op Fl e Ar maxbpg
.Op Fl m Ar minfree
.Bk -words
.Op Fl o Ar optimize_preference
.Ek
.Op Ar special | Ar filesys
.Sh DESCRIPTION
.Nm Tunefs
is designed to change the dynamic parameters of a file system
which affect the layout policies.
The parameters which are to be changed are indicated by the flags
given below:
.Bl -tag -width Ds
.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 Ar maxcontig
This specifies the maximum number of contiguous blocks that will
be laid out before forcing a rotational delay (see
.Fl d
below).
The default value is one, since most device drivers require
an interrupt per disk transfer.
Device drivers that can chain several buffers together in a single
transfer should set this to the maximum chain length.
.It Fl d Ar rotdelay
This specifies the expected time (in milliseconds)
to service a transfer completion
interrupt and initiate a new transfer on the same disk.
It is used to decide how much rotational spacing to place between
successive blocks in a file.
.It Fl e Ar maxbpg
This indicates 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 m Ar minfree
This value specifies the percentage of space held back
from normal users; the minimum free space threshold.
The default value used is 10%.
This value can be set to zero, however up to a factor of three
in throughput will be lost over the performance obtained at a 10%
threshold.
Note that 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 o Ar optimize_preference
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.
If the value of minfree (see above) is less than 10%,
then the file system should optimize for space to avoid
running out of full sized blocks.
For values of minfree greater than or equal to 10%,
fragmentation is unlikely to be problematical, and
the file system can be optimized for time.
.It Fl p
This option shows a summary of what the current tuneable settings
are on the selected file system.  More detailed information can be
obtained in the
.Xr dumpfs 8
manual page.
.El
.Sh SEE ALSO
.Xr fs 5 ,
.Xr dumpfs 8 ,
.Xr newfs 8 ,
.Xr mkfs 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 BUGS
This program should work on mounted and active file systems.
Because the super-block is not kept in the buffer cache,
the changes will only take effect if the program
is run on dismounted file systems.
To change the root file system, the system must be rebooted
after the file system is tuned.
.Pp
.\" Take this out and a Unix Demon will dog your steps from now until
.\" the time_t's wrap around.
You can tune a file system, but you can't tune a fish.
.Sh HISTORY
The
.Nm
command appeared in
.Bx 4.2 .