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32 .\" @(#)tunefs.8 8.2 (Berkeley) 12/11/93
40 .Nd tune up an existing file system
47 .Op Fl f Ar avgfilesize
50 .Op Fl n Ar enable | disable
51 .Op Fl o Ar space | time
55 .Ar special | Ar filesystem
58 is designed to change the dynamic parameters of a file system
59 which affect the layout policies.
60 The parameters which are to be changed are indicated by the flags
62 .Bl -tag -width indent
64 The file system has several backups of the super-block. Specifying
65 this option will cause all backups to be modified as well as the
66 primary super-block. This is potentially dangerous - use with caution.
68 Specify the maximum number of contiguous blocks that will
69 be laid out before forcing a rotational delay (see
72 The default value is one, since most device drivers require
73 an interrupt per disk transfer.
74 Device drivers that can chain several buffers together in a single
75 transfer should set this to the maximum chain length.
77 Specify the expected time (in milliseconds)
78 to service a transfer completion
79 interrupt and initiate a new transfer on the same disk.
80 It is used to decide how much rotational spacing to place between
81 successive blocks in a file.
83 Indicate the maximum number of blocks any single file can
84 allocate out of a cylinder group before it is forced to begin
85 allocating blocks from another cylinder group.
86 Typically this value is set to about one quarter of the total blocks
88 The intent is to prevent any single file from using up all the
89 blocks in a single cylinder group,
90 thus degrading access times for all files subsequently allocated
91 in that cylinder group.
92 The effect of this limit is to cause big files to do long seeks
93 more frequently than if they were allowed to allocate all the blocks
94 in a cylinder group before seeking elsewhere.
95 For file systems with exclusively large files,
96 this parameter should be set higher.
97 .It Fl f Ar avgfilezsize
98 Specify the expected average file size.
100 Specify the percentage of space held back
101 from normal users; the minimum free space threshold.
102 The default value used is 8%.
103 This value can be set to zero, however up to a factor of three
104 in throughput will be lost over the performance obtained at a 10%
106 Settings of 5% and less force space optimization to
107 always be used which will greatly increase the overhead for file
109 Note that if the value is raised above the current usage level,
110 users will be unable to allocate files until enough files have
111 been deleted to get under the higher threshold.
112 .It Fl n Ar enable | disable
113 Turn on/off soft updates.
114 An unmounted filesystem is required.
115 .It Fl o Ar space | time
116 The file system can either try to minimize the time spent
117 allocating blocks, or it can attempt to minimize the space
118 fragmentation on the disk.
119 Optimization for space has much
120 higher overhead for file writes.
121 The kernel normally changes the preference automatically as
122 the percent fragmentation changes on the file system.
124 Show a summary of what the current tunable settings
125 are on the selected file system. More detailed information can be
130 Specify the expected number of files per directory.
133 At least one of the above flags is required.
143 .%T "A Fast File System for UNIX"
144 .%J "ACM Transactions on Computer Systems 2"
148 .%O "(reprinted in the BSD System Manager's Manual, SMM:5)"
151 This program should work on mounted and active file systems.
152 Because the super-block is not kept in the buffer cache,
153 the changes will only take effect if the program
154 is run on dismounted file systems.
155 To change the root file system, the system must be rebooted
156 after the file system is tuned.
157 .\" Take this out and a Unix Demon will dog your steps from now until
158 .\" the time_t's wrap around.
160 You can tune a file system, but you can't tune a fish.