TUNEFS(8) BSD System Manager's Manual TUNEFS(8)
tunefs - tune up an existing file system
tunefs [-Ap] [-a maxcontig] [-d rotdelay] [-e maxbpg] [-f avgfilesize] [-m minfree] [-n avgfpdir] [-o optimize_preference] [special | filesys]
tunefs is designed to change the dynamic parameters of a file system which affect the layout policies. The parameters which are to be changed are indicated by the flags given below: -A The file system has several backups of the super-block. Specify- ing this option will cause all backups to be modified as well as the primary super-block. This is potentially dangerous - use with caution. -a maxcontig This specifies the maximum number of contiguous blocks that will be laid out before forcing a rotational delay (see -d below). The default value depends on the block size (4 for 16KB blocks, 8 for 8KB blocks and 16 for 4KB blocks). Most device drivers can chain several buffers together in a single transfer. For optimal per- formance, the value of maxcontig should be chosen based on the maximum chain length supported by the device driver. -d rotdelay This specifies the expected time (in milliseconds) to service a transfer completion interrupt and initiate a new transfer on the same disk. It is used to decide how much rotational spacing to place between successive blocks in a file. -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 al- locating blocks from another cylinder group. Typically this value is set to about one quarter of the total blocks in a cylinder 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 cylinder 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. -f avgfilesize Specifies the expected average file size in bytes. This value could be used for various optimizations, but for now it is only used together with avgfpdir to optimize the directory allocation policy. To take effect, both avgfpdir and avgfilesize must be greater than zero. (Also see avgfpdir.) -m minfree This value specifies the percentage of space held back from nor- mal users; the minimum free space threshold. The default value used is 5%. This value can be set to zero; however, a factor of up to three in throughput will be lost over the performance ob- tained 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. -n avgfpdir Specifies the expected average number of files per directory in the file system. This value is used only if both avgfilesize and avgfpdir are greater than zero. It is used to limit number of directories which may be allocated one after another in the same cylinder group without intervention by regular files. This value does not affect most file system operations but is useful for ap- plications which at first create a directory structure and then populate with files later. (Also see avgfilesize.) -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. -p This option shows a summary of what the current tuneable settings are on the selected file system. More detailed information can be obtained in the dumpfs(8) manual page.
fs(5), dumpfs(8), growfs(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 dismounted 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. MirOS BSD #10-current December 11, 1993 1
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