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32 .\" @(#)tunefs.8 8.2 (Berkeley) 12/11/93
40 .Nd tune up an existing filesystem
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 filesystem
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 filesystem 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 filesystems 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 Note that lowering the threshold can adversely affect performance:
107 Settings of 5% and less force space optimization to
108 always be used which will greatly increase the overhead for file
111 The filesystem's ability to avoid fragmentation will be reduced
112 when the total free space, including the reserve, drops below 15%.
113 As free space approaches zero, throughput can degrade by up to a
114 factor of three over the performance obtained at a 10% threshold.
117 If the value is raised above the current usage level,
118 users will be unable to allocate files until enough files have
119 been deleted to get under the higher threshold.
120 .It Fl n Ar enable | disable
121 Turn on/off soft updates.
122 An unmounted filesystem is required.
123 .It Fl o Ar space | time
124 The filesystem can either try to minimize the time spent
125 allocating blocks, or it can attempt to minimize the space
126 fragmentation on the disk.
127 Optimization for space has much
128 higher overhead for file writes.
129 The kernel normally changes the preference automatically as
130 the percent fragmentation changes on the filesystem.
132 Show a summary of what the current tunable settings
133 are on the selected filesystem. More detailed information can be
138 Specify the expected number of files per directory.
141 At least one of the above flags is required.
143 .Bl -tag -width /etc/fstab -compact
145 read this to determine the device file for a
146 specified mount point.
157 .%T "A Fast File System for UNIX"
158 .%J "ACM Transactions on Computer Systems 2"
162 .%O "(reprinted in the BSD System Manager's Manual, SMM:5)"
165 This program should work on mounted and active filesystems.
166 Because the super-block is not kept in the buffer cache,
167 the changes will only take effect if the program
168 is run on dismounted filesystems.
169 To change the root filesystem, the system must be rebooted
170 after the filesystem is tuned.
171 .\" Take this out and a Unix Demon will dog your steps from now until
172 .\" the time_t's wrap around.
174 You can tune a filesystem, but you can't tune a fish.