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28 .\" @(#)tunefs.8 8.2 (Berkeley) 12/11/93
36 .Nd tune up an existing file system
40 .Op Fl a Cm enable | disable
42 .Op Fl f Ar avgfilesize
43 .Op Fl J Cm enable | disable
45 .Op Fl l Cm enable | disable
47 .Op Fl n Cm enable | disable
48 .Op Fl o Cm space | time
51 .Ar special | filesystem
55 utility is designed to change the dynamic parameters of a file system
56 which affect the layout policies.
59 utility cannot be run on an active file system.
60 To change an active file system,
61 it must be downgraded to read-only or unmounted.
63 The parameters which are to be changed are indicated by the flags
65 .Bl -tag -width indent
67 The file system has several backups of the super-block.
69 this option will cause all backups to be modified as well as the
71 This is potentially dangerous - use with caution.
72 .It Fl a Cm enable | disable
73 Turn on/off the administrative ACL enable flag.
75 Indicate the maximum number of blocks any single file can
76 allocate out of a cylinder group before it is forced to begin
77 allocating blocks from another cylinder group.
78 Typically this value is set to about one quarter of the total blocks
80 The intent is to prevent any single file from using up all the
81 blocks in a single cylinder group,
82 thus degrading access times for all files subsequently allocated
83 in that cylinder group.
84 The effect of this limit is to cause big files to do long seeks
85 more frequently than if they were allowed to allocate all the blocks
86 in a cylinder group before seeking elsewhere.
87 For file systems with exclusively large files,
88 this parameter should be set higher.
89 .It Fl f Ar avgfilesize
90 Specify the expected average file size.
91 .It Fl J Cm enable | disable
92 Turn on/off GJournal flag.
94 Add/modify an optional file system volume label.
95 .It Fl l Cm enable | disable
96 Turn on/off MAC multilabel flag.
98 Specify the percentage of space held back
99 from normal users; the minimum free space threshold.
100 The default value used is 8%.
101 Note that lowering the threshold can adversely affect performance:
104 Settings of 5% and less force space optimization to
105 always be used which will greatly increase the overhead for file
108 The file system's ability to avoid fragmentation will be reduced
109 when the total free space, including the reserve, drops below 15%.
110 As free space approaches zero, throughput can degrade by up to a
111 factor of three over the performance obtained at a 10% threshold.
114 If the value is raised above the current usage level,
115 users will be unable to allocate files until enough files have
116 been deleted to get under the higher threshold.
117 .It Fl n Cm enable | disable
118 Turn on/off soft updates.
119 .It Fl o Cm space | time
120 The file system can either try to minimize the time spent
121 allocating blocks, or it can attempt to minimize the space
122 fragmentation on the disk.
123 Optimization for space has much
124 higher overhead for file writes.
125 The kernel normally changes the preference automatically as
126 the percent fragmentation changes on the file system.
128 Show a summary of what the current tunable settings
129 are on the selected file system.
130 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.
154 .%T "A Fast File System for UNIX"
155 .%J "ACM Transactions on Computer Systems 2"
159 .%O "(reprinted in the BSD System Manager's Manual, SMM:5)"
167 This utility should work on active file systems.
168 To change the root file system, the system must be rebooted
169 after the file system is tuned.
170 .\" Take this out and a Unix Daemon will dog your steps from now until
171 .\" the time_t's wrap around.
173 You can tune a file system, but you cannot tune a fish.