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32 .\" @(#)tunefs.8 8.2 (Berkeley) 12/11/93
40 .Nd tune up an existing filesystem
46 .Op Fl f Ar avgfilesize
49 .Op Fl n Ar enable | disable
50 .Op Fl o Ar space | time
54 .Ar special | Ar filesystem
58 utility is designed to change the dynamic parameters of a filesystem
59 which affect the layout policies.
62 program cannot be run on an active filesystem.
63 To change an active filesystem,
64 you must either downgrade the filesystem to read-only
67 The parameters which are to be changed are indicated by the flags
69 .Bl -tag -width indent
71 The filesystem has several backups of the super-block. Specifying
72 this option will cause all backups to be modified as well as the
73 primary super-block. This is potentially dangerous - use with caution.
75 Specify the maximum number of contiguous blocks that will
76 be laid out before allowing a rotational delay.
77 The default value is 16.
79 Indicate the maximum number of blocks any single file can
80 allocate out of a cylinder group before it is forced to begin
81 allocating blocks from another cylinder group.
82 Typically this value is set to about one quarter of the total blocks
84 The intent is to prevent any single file from using up all the
85 blocks in a single cylinder group,
86 thus degrading access times for all files subsequently allocated
87 in that cylinder group.
88 The effect of this limit is to cause big files to do long seeks
89 more frequently than if they were allowed to allocate all the blocks
90 in a cylinder group before seeking elsewhere.
91 For filesystems with exclusively large files,
92 this parameter should be set higher.
93 .It Fl f Ar avgfilezsize
94 Specify the expected average file size.
96 Specify the percentage of space held back
97 from normal users; the minimum free space threshold.
98 The default value used is 8%.
99 Note that lowering the threshold can adversely affect performance:
103 Settings of 5% and less force space optimization to
104 always be used which will greatly increase the overhead for file
107 The filesystem's ability to avoid fragmentation will be reduced
108 when the total free space, including the reserve, drops below 15%.
109 As free space approaches zero, throughput can degrade by up to a
110 factor of three over the performance obtained at a 10% threshold.
113 If the value is raised above the current usage level,
114 users will be unable to allocate files until enough files have
115 been deleted to get under the higher threshold.
116 .It Fl n Ar enable | disable
117 Turn on/off soft updates.
118 .It Fl o Ar space | time
119 The filesystem can either try to minimize the time spent
120 allocating blocks, or it can attempt to minimize the space
121 fragmentation on the disk.
122 Optimization for space has much
123 higher overhead for file writes.
124 The kernel normally changes the preference automatically as
125 the percent fragmentation changes on the filesystem.
127 Show a summary of what the current tunable settings
128 are on the selected filesystem. More detailed information can be
135 Specify the expected number of files per directory.
138 At least one of the above flags is required.
140 .Bl -tag -width ".Pa /etc/fstab"
142 read this to determine the device file for a
143 specified mount point.
155 .%T "A Fast File System for UNIX"
156 .%J "ACM Transactions on Computer Systems 2"
160 .%O "(reprinted in the BSD System Manager's Manual, SMM:5)"
163 This program should work on active filesystems.
164 .\" Take this out and a Unix Daemon will dog your steps from now until
165 .\" the time_t's wrap around.
167 You can tune a filesystem, but you can't tune a fish.