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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 n Cm enable | disable
49 .Op Fl o Cm space | time
52 .Ar special | filesystem
56 utility is designed to change the dynamic parameters of a file system
57 which affect the layout policies.
60 utility cannot be run on an active file system.
61 To change an active file system,
62 it must be downgraded to read-only or unmounted.
64 The parameters which are to be changed are indicated by the flags
66 .Bl -tag -width indent
68 The file system has several backups of the super-block.
70 this option will cause all backups to be modified as well as the
72 This is potentially dangerous - use with caution.
73 .It Fl a Cm enable | disable
74 Turn on/off the administrative POSIX.1e ACL enable flag.
76 Indicate the maximum number of blocks any single file can
77 allocate out of a cylinder group before it is forced to begin
78 allocating blocks from another cylinder group.
79 Typically this value is set to about one quarter of the total blocks
81 The intent is to prevent any single file from using up all the
82 blocks in a single cylinder group,
83 thus degrading access times for all files subsequently allocated
84 in that cylinder group.
85 The effect of this limit is to cause big files to do long seeks
86 more frequently than if they were allowed to allocate all the blocks
87 in a cylinder group before seeking elsewhere.
88 For file systems with exclusively large files,
89 this parameter should be set higher.
90 .It Fl f Ar avgfilesize
91 Specify the expected average file size.
92 .It Fl J Cm enable | disable
93 Turn on/off gjournal flag.
95 Add/modify an optional file system volume label.
96 .It Fl l Cm enable | disable
97 Turn on/off MAC multilabel flag.
99 Specify the percentage of space held back
100 from normal users; the minimum free space threshold.
101 The default value used is 8%.
102 Note that lowering the threshold can adversely affect performance:
105 Settings of 5% and less force space optimization to
106 always be used which will greatly increase the overhead for file
109 The file system's ability to avoid fragmentation will be reduced
110 when the total free space, including the reserve, drops below 15%.
111 As free space approaches zero, throughput can degrade by up to a
112 factor of three over the performance obtained at a 10% threshold.
115 If the value is raised above the current usage level,
116 users will be unable to allocate files until enough files have
117 been deleted to get under the higher threshold.
118 .It Fl N Cm enable | disable
119 Turn on/off the administrative NFSv4 ACL enable flag.
120 .It Fl n Cm enable | disable
121 Turn on/off soft updates.
122 .It Fl o Cm space | time
123 The file system can either try to minimize the time spent
124 allocating blocks, or it can attempt to minimize the space
125 fragmentation on the disk.
126 Optimization for space has much
127 higher overhead for file writes.
128 The kernel normally changes the preference automatically as
129 the percent fragmentation changes on the file system.
131 Show a summary of what the current tunable settings
132 are on the selected file system.
133 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 ".Pa /etc/fstab"
145 read this to determine the device file for a
146 specified mount point.
158 .%T "A Fast File System for UNIX"
159 .%J "ACM Transactions on Computer Systems 2"
163 .%O "(reprinted in the BSD System Manager's Manual, SMM:5)"
171 This utility should work on active file systems.
172 To change the root file system, the system must be rebooted
173 after the file system is tuned.
174 .\" Take this out and a Unix Daemon will dog your steps from now until
175 .\" the time_t's wrap around.
177 You can tune a file system, but you cannot tune a fish.