4.2BSD/usr/man/man8/tunefs.8

.TH TUNEFS 8 "20 February 1983"
.UC 4
.SH NAME
tunefs \- tune up an existing file system
.SH SYNOPSIS
.B /etc/tunefs
.I tuneup-options
.IR special | filesys
.SH DESCRIPTION
.I 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:
.IP "\fB\-a\fP maxcontig"
.br
This specifies the maximum number of contiguous blocks that will
be laid out before forcing a rotational delay (see \-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.
.IP "\fB\-d\fP rotdelay"
.br
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.
.IP "\fB\-e\fP maxbpg"
.br
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.
.IP "\fB\-m\fP minfree"
.br
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.
.SH "SEE ALSO"
fs(5),
newfs(8),
mkfs(8)
.PP
McKusick, Joy, Leffler; "A Fast File System for Unix",
Computer Systems Research Group, Dept of EECS, Berkeley, CA 94720;
TR #7, September 1982.
.SH BUGS
This program should work on mounted and active file systems.
Because the super-block is not kept in the buffer cache,
the program will only take effect if it is run on dismounted file systems.
(if run on the root file system, the system must be rebooted)
.PP
You can tune a file system, but you can't tune a fish.