TUNEFS(8)               NetBSD System Manager's Manual               TUNEFS(8)

tunefs -- tune up an existing file system
tunefs [-AFN] [-e maxbpg] [-g avgfilesize] [-h avgfpdir] [-l logsize] [-m minfree] [-o optimize_preference] special | filesys
tunefs is designed to change the dynamic parameters of a file system which affect the layout policies. The following options are supported by tunefs: -A Cause the values to be updated in all the alternate superblocks instead of just the standard superblock. If this option is not used, then use of a backup superblock by fsck(8) will lose any- thing changed by tunefs. -A is ignored when -N is specified. -F Indicates that special is a file system image, rather than a device name or file system mount point. special will be accessed `as-is'. -N Display all the settable options (after any changes from the tun- ing options) but do not cause any of them to be changed. -e maxbpg This indicates the maximum number of blocks any single file can allocate out of a cylinder group before it is forced to begin allocating blocks from another cylinder group. Typically this value is set to about one quarter of the total blocks in a cylin- der group. The intent is to prevent any single file from using up all the blocks in a single cylinder group, thus degrading access times for all files subsequently allocated in that cylin- der group. The effect of this limit is to cause big files to do long seeks more frequently than if they were allowed to allocate all the blocks in a cylinder group before seeking elsewhere. For file systems with exclusively large files, this parameter should be set higher. -g avgfilesize This specifies the expected average file size. -h avgfpdir This specifies the expected number of files per directory. -l logsize This value specifies the size of the in-filesystem journaling log file. The default journaling log file size is described in wapbl(4). Specifying a size of zero will cause the in-filesystem journaling log file to be removed the next time the filesystem is mounted. The size of an existing in-filesystem journaling log file can not be changed. -m minfree This value specifies the percentage of space held back from nor- mal users; the minimum free space threshold. The default value is set during creation of the filesystem, see newfs(8). This value can be set to zero, however up to a factor of three in throughput will be lost over the performance obtained at a 5% threshold. Note that if the value is raised above the current usage level, users will be unable to allocate files until enough files have been deleted to get under the higher threshold. -o optimize_preference The file system can either try to minimize the time spent allo- cating blocks, or it can attempt to minimize the space fragmenta- tion on the disk. If the value of minfree (see above) is less than 5%, then the file system should optimize for space to avoid running out of full sized blocks. For values of minfree greater than or equal to 5%, fragmentation is unlikely to be problemati- cal, and the file system can be optimized for time. optimize_preference can be specified as either space or time.
wapbl(4), fs(5), dumpfs(8), fsck_ffs(8), newfs(8) M. McKusick, W. Joy, S. Leffler, and R. Fabry, "A Fast File System for UNIX", ACM Transactions on Computer Systems 2, 3, pp 181-197, August 1984, (reprinted in the BSD System Manager's Manual, SMM:5).
The tunefs command appeared in 4.2BSD.
This program should work on mounted and active file systems. Because the super-block is not kept in the buffer cache, the changes will only take effect if the program is run on unmounted file systems. To change the root file system, the system must be rebooted after the file system is tuned. You can tune a file system, but you can't tune a fish. NetBSD 5.0 April 26, 2004 NetBSD 5.0

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