tunefs - tune up an existing HFS file system
/usr/sbin/tunefs [-A] [-v] [-a maxcontig] [-d rotdelay]
[-e maxbpg] [-m minfree]
[-r advanced read-ahead] special-device
The tunefs command is used to alter dynamic parameters that affect HFS
file system layout policies. Parameters to be altered are specified
by the options and arguments provided on the command line as described
tunefs affects how the file system blocks are laid out on the disk.
The default rotdelay value set by the newfs and mkfs commands (see
newfs(1M) and mkfs(1M)) is 0 milliseconds, causing file system blocks
to be written and read consecutively. In general, this should be the
optimal tuning, making the use of tunefs -d unnecessary.
tunefs recognizes the following options and command-line arguments:
-a maxcontig Set the maximum number of contiguous blocks that
will be laid out before forcing a rotational delay
to maxcontig (see -d below). The default value is
1, because most device drivers require one
interrupt per disk transfer. For device drivers
that can chain several buffers together in a
single transfer, set maxcontig to the maximum
-d rotdelay rotdelay is the expected time (in milliseconds) to
service a transfer completion interrupt and
initiate a new transfer on the same disk. It is
used to determine how much rotational spacing to
place between successive blocks in a file.
-e maxbpg maxbpg specifies 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 fourth 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 large files to do long seeks more frequently
than if they were allowed to allocate all the
blocks in a cylinder group before seeking
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elsewhere. For file systems with exclusively
large files, this parameter should be set higher.
-m minfree minfree specifies the percentage of space that is
not available to normal users; i.e., the minimum
free space threshold. The default value used is
10%. This value can be set to zero. If set to
zero, throughput performance drops to as little as
one-third of the efficiency expected when the
threshold is set at 10%. Note that if minfree is
raised above the current usage level, users cannot
allocate files until enough files have been
deleted to meet the new threshold requirement.
-r advanced read-ahead
Advanced read-ahead specifies whether the file
system should use an advanced predictive read-
ahead algorithm. The implementation requires more
system resources in exchange for an advanced
access pattern recognition. Patterns include
forward sequential, backward sequential, forward
strided, and backward strided. This value can be
set to zero (disable) or one (enable). By
default, a file system will have advanced read-
ahead enabled when created.
-v (visual) Display current values contained in the
primary super-block to standard output.
-A (all) Modify redundant super-blocks as well as the
primary super-block as stipulated by the
configuration options and arguments.
special-device is the name of the file system to be tuned. It is
either a block or character special file if the
file system is not mounted, or a block special
file if the file system is mounted.
Root file system tuning is normally done during initial system
software installation. Tuning the root file system after installation
has little useful effect because so many files have already been
You can tune a file system, but you can't tune a fish.
tunefs was developed by the University of California, Berkeley.
dumpfs(1M), mkfs(1M), newfs(1M), fs(4).
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