tunefs(1M)							    tunefs(1M)

NAME
       tunefs - tune up an existing HFS file system

SYNOPSIS
       maxcontig] rotdelay] maxbpg] minfree]
	      advanced read-ahead] special-device

DESCRIPTION
       The  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 below.

       affects	how  the  file	system	blocks	are laid out on the disk.  The
       default rotdelay value set by  the  and	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 unnecessary.

   Options
       recognizes the following options and command-line arguments:

	      Set  the	maximum	 number of contiguous blocks that will be laid
	      out
			     before forcing a rotational  delay	 to  maxcontig
			     (see  below).   The default value is because most
			     device drivers require  one  interrupt  per  disk
			     transfer.	For device drivers that can chain sev‐
			     eral buffers together in a single	transfer,  set
			     maxcontig to the maximum chain length.

	      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.

	      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 fre‐
			     quently 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.

	      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 thresh‐
			     old  is  set  at  10%.   Note  that if minfree is
			     raised above the current usage level, users  can‐
			     not  allocate  files until enough files have been
			     deleted to meet the new threshold requirement.

	      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 recogni‐
			     tion.  Patterns include forward sequential, back‐
			     ward  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.

	      (visual) Display current values
			     contained in the primary super-block to  standard
			     output.

	      (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 spe‐
			     cial file if the file system is mounted.

WARNINGS
       Root file system tuning is normally done during initial system software
       installation.   Tuning the root file system after installation has lit‐
       tle useful effect because so many files have already been written.

AUTHOR
       was developed by the University of California, Berkeley.

SEE ALSO
       dumpfs(1M), mkfs(1M), newfs(1M).

								    tunefs(1M)

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