vxtunefs(1M)															      vxtunefs(1M)

NAME

       vxtunefs - tune a VxFS File System

SYNOPSIS

       tunefstab] parameter=value] [{mount_point|block_special}]...

DESCRIPTION

       sets or prints tuneable I/O parameters of mounted file systems.	can set parameters describing the I/O properties of the underlying device,
       parameters to indicate when to treat an I/O as direct I/O, or parameters to control the extent allocation policy  for  the  specified  file
       system.

       With no options specified, prints the existing VxFS parameters for the specified file systems.

       works on a list of mount points specified on the command line, or all the mounted file systems listed in the file.  The default file is You
       can change the default using the option.

       can be run at any time on a mounted file system, and all parameter changes take immediate effect.  Parameters specified on the command line
       override parameters listed in the file.

       If exists, the VxFS-specific command invokes to set device parameters from

       If the file system is built on a VERITAS Volume Manager (VxVM) volume, the VxFS-specific command interacts with VxVM to obtain default val-
       ues for the tunables, so you need to specify tunables for VxVM devices only to change the defaults.

       Only a privileged user can run

   Options
       recognizes the following options:

	      Use	     filename instead of as the file containing tuning parameters.

	      Specify parameters for the file systems
			     listed on the command line.  See the "VxFS Tuning Parameters and Guidelines" topic in this section.

	      Print the tuning parameters for
			     all the file systems specified on the command line.

	      Set the new tuning parameters for the VxFS file systems
			     specified on the command line or in the file.

   Operands
       recognizes the following operands:

	      mount_point    Name of directory for a mounted VxFS file system.

	      block_special  Name of the block_special device which contains the VxFS file system.

   Notes
       works with Storage Checkpoints; however, VxFS tunables apply to an entire file system.  Therefore tunables  affect  not	only  the  primary
       fileset, but also any Storage Checkpoint filesets within that file system.

       The tunables and are not supported on HP-UX.

   VxFS Tuning Parameters and Guidelines
       The values for all the following parameters except and can be specified in bytes, kilobytes, megabytes or sectors (1024 bytes) by appending
       or You do not need for a suffix for the value in bytes.

       If the file system is being used with a hardware disk array or another volume manager (such as VxVM), align the	parameters  to	match  the
       geometry  of the logical disk.  For disk striping and RAID-5 configurations, set to the stripe unit size or interleave factor and set to be
       the number of columns.  For disk striping configurations, set and to the same values as and but for RAID-5 configurations, set to the  full
       stripe size and set to 1.

       For  an application to do efficient direct I/O or discovered direct I/O, it should issue read requests that are equal to the product of and
       In general, any multiple or factor of multiplied by is a good size for performance.  For writing, the same general rule applies to the  and
       parameters.  When tuning a file system, the best thing to do is use the tuning parameters under a real workload.

       If  an  application is doing sequential I/O to large files, it should issue requests larger than the This performs the I/O requests as dis-
       covered direct I/O requests which are unbuffered like direct I/O, but which do not require synchronous inode  updates  when  extending  the
       file.   If the file is too large to fit in the cache, using unbuffered I/O avoids losing useful data out of the cache, and lowers CPU over-
       head.

       The VxFS tuneable parameters are:

       On VxFS, files can have up to 10 variable
	    sized extents stored in the inode.	After these extents are used, the file must use indirect extents which are a fixed  size  that	is
	    set  when  the file first uses indirect extents.  These indirect extents are 8K by default.  The file system does not use larger indi-
	    rect extents because it must fail a write and return ENOSPC if there are no extents available that are the indirect extent size.   For
	    file  systems  with many large files, the 8K indirect extent size is too small.  The files that get into indirect extents use a lot of
	    smaller extents instead of a few larger ones.  By using this parameter, the default indirect extent size  can  be  increased  so  that
	    large files in indirects use fewer larger extents.

	    Be	careful  using this tuneable. If it is too large, then writes fail when they are unable to allocate extents of the indirect extent
	    size to a file.  In general, the fewer and the larger the files on a file system, the larger can be.  The value of this  parameter	is
	    generally a multiple of the parameter.

	    This tuneable does not apply to disk layout Version 4.

       Any file I/O requests larger than the
	    are  handled  as  discovered direct I/O.  A discovered direct I/O is unbuffered like direct I/O, but it does not require a synchronous
	    commit of the inode when the file is extended or blocks are allocated.  For larger I/O requests, the CPU time  for	copying  the  data
	    into  the buffer cache and the cost of using memory to buffer the I/O becomes more expensive than the cost of doing the disk I/O.  For
	    these I/O requests, using discovered direct I/O is more efficient than regular I/O.  The default value of this parameter is 256K.

       Specifies the minimum amount of time,
	    in seconds, that the VxFS File Change Log (FCL) keeps records in the log.  When the oldest 8K block of  FCL  records  have	been  kept
	    longer  than  the  value  of  they	are  purged from the FCL and the extents nearest to the beginning of the FCL file are freed.  This
	    process is referred to as "punching a hole."  Holes are punched in the FCL file in 8K chunks.

	    If the parameter is set, records are purged from the FCL if the amount of space allocated to the FCL exceeds even if the elapsed  time
	    the  records have been in the log is less than the value of If the file system runs out of space before is reached, the FCL is deacti-
	    vated.

	    Either or both of the or parameters must be set before the File Change Log can be activated.  operates only on  Version  6	or  higher
	    disk layout file systems.

       Specifies the maximum amount of space that can be allocated to the
	    VxFS  File Change Log.  The FCL file is a sparse file that grows as changes occur in the file system.  When the space allocated to the
	    FCL file reaches the value, the oldest FCL records are purged from the FCL and the extents nearest to the beginning of  the  FCL  file
	    are  freed.   This	process  is referred to as "punching a hole."  Holes are punched in the FCL file in 8K chunks.	If the file system
	    runs out of space before is reached, the FCL is deactivated.

	    Either or both of the or parameters must be set before the File Change Log can be activated.  operates only on  Version  6	or  higher
	    disk layout file systems.

       Specifies the time,
	    in	seconds,  that	must elapse before the VxFS File Change Log records a data overwrite, data extending write, or data truncate for a
	    file.  The ability to limit the number of repetitive FCL records for continuous writes to the same file is important for  file  system
	    performance  and for applications processing the FCL.  is best set to an interval less than the shortest interval between reads of the
	    FCL by any application.  This way all applications using the FCL can be assured of finding at least one FCL record for any file  expe-
	    riencing continuous data changes.

	    is	enforced  for  all files in the file system.  Each file maintains its own time stamps, and the elapsed time between FCL records is
	    per file.  This elapsed time can be overridden using the VxFS FCL sync public API (see the manual page).

	    operates only on Version 6 or higher disk layout file systems.

       For a file managed by a hierarchical storage management
	    (HSM) application, preallocates disk blocks before data is migrated back into the file system.  An HSM  application  usually  migrates
	    the  data  back  through a series of writes to the file, each of which allocates a few blocks.  By setting a sufficient number of disk
	    blocks will be allocated on the first write to the empty file so that no disk block allocation  is	required  for  subsequent  writes,
	    which improves the write performance during migration.

	    The  parameter  is	implemented outside of the DMAPI specification, and its usage has limitations depending on how the space within an
	    HSM controlled file is managed.  It is advisable to use only when recommended by the HSM application controlling the file system.

       Changes the default size of the initial extent.

	    VxFS determines, based on the first write to a new file, the size of the first extent to allocate to the  file.  Typically	the  first
	    extent  is	the  smallest  power  of  2 that is larger than the size of the first write. If that power of 2 is less than 8K, the first
	    extent allocated is 8K. After the initial extent, the file system increases the size of subsequent extents (see with each allocation.

	    For a file managed by a hierarchical storage management (HSM) application, preallocates disk blocks before data is migrated back  into
	    the  file system.  An HSM application usually migrates the data back through a series of writes to the file, each of which allocates a
	    few blocks.  By setting a sufficient number of disk blocks will be allocated on the first write to the empty  file	so  that  no  disk
	    block allocation is required for subsequent writes, which improves the write performance during migration.

	    Because most applications write to files using a buffer size of 8K or less, the increasing extents start doubling from a small initial
	    extent.  changes the default initial extent size to a larger value, so the doubling policy starts from a much larger initial size, and
	    the file system won't allocate a set of small extents at the start of file.

	    Use  this  parameter  only on file systems that have a very large average file size. On such file systems, there are fewer extents per
	    file and less fragmentation.

	    is measured in file system blocks.

       Specifies the maximum number of inodes to
	    place on an inode aging list.  Inode aging is used in conjunction with file system Storage Checkpoints to allow quick  restoration	of
	    large,  recently deleted files.  The aging list is maintained in first-in-first-out (fifo) order up to maximum number of inodes speci-
	    fied by As newer inodes are placed on the list, older inodes are removed to complete their aging process.  For best performance, it is
	    advisable to age only a limited number of larger files before completion of the removal process.  The default maximum number of inodes
	    to age is 2048.

       Specifies the minimum size to qualify a deleted inode for inode aging.
	    Inode aging is used in conjunction with file system Storage Checkpoints to allow quick restoration of large, recently  deleted  files.
	    For  best performance, it is advisable to age only a limited number of larger files before completion of the removal process.  Setting
	    the size too low can push larger file inodes out of the aging queue to make room for newly removed smaller file inodes.

       Determines the maximum buffer size allocated for file data.
	    The two accepted values are 8K bytes and 64K bytes.  The larger value can be beneficial for workloads  where  large  reads/writes  are
	    performed sequentially.  The smaller value is preferable on workloads where the I/O is random or is done in small chunks.  The default
	    value is 8K bytes.

       Maximum size of a direct I/O
	    request issued by the file system.	If there is a larger I/O request, it is broken up into chunks.	This parameter	defines  how  much
	    memory an I/O request can lock at once; do not set it to more than 20% of memory.

       Limits the maximum disk queue generated by a single file.
	    When  the  file  system  is flushing data for a file and the number of pages being flushed exceeds processes block until the amount of
	    data being flushed decreases.  Although this does not limit the actual disk queue, it prevents synchronizing processes from making the
	    system unresponsive. The default value is 1 megabyte.

	    Although it does not limit the actual disk queue, prevents processes that flush data to disk, such as from making the system unrespon-
	    sive.

	    See the description for more information on pages and system memory.

       Increases or decreases the maximum size of an extent.
	    When the file system is following its default allocation policy for sequential writes to a file, it allocates an initial  extent  that
	    is	large  enough for the first write to the file. When additional extents are allocated, they are progressively larger (the algorithm
	    tries to double the size of the file with each new extent), so each extent can hold several writes worth of  data.	This  reduces  the
	    total  number  of  extents	in anticipation of continued sequential writes. When there are no more writes to the file, unused space is
	    freed for other files to use.

	    In general, this allocation stops increasing the size of extents at 2048 blocks, which prevents one file from holding too much  unused
	    space.

	    is measured in file system blocks.

	    is	measured in file system blocks. The default value for this tunable is 2048 blocks. Setting to a value less than 2048 automatically
	    resets this tunable to the default value of 2048 blocks.

       Enables or disables caching on Quick I/O for Databases files.
	    The default behavior is to disable caching.  To enable caching, set to 1.

	    On systems with large amounts of memory, the database cannot always use all of the memory as a cache.  By enabling file system caching
	    as a second level cache, performance can improve.

	    If	the database is performing sequential scans of tables, the scans can run faster by enabling file system caching so the file system
	    performs aggressive read aheads on the files.

       In the absence of a specific caching advisory,
	    the default for all VxFS read operations is to perform sequential read ahead.  The enhanced read  ahead  functionality  implements	an
	    algorithm  that  allows read aheads to detect more elaborate patterns (such as increasing or decreasing read offsets, or multithreaded
	    file accesses) in addition to simple sequential reads.  You can specify the following values for

	    Disables read ahead functionality
	    Retains traditional sequential read ahead behavior
	    Enables enhanced read ahead for all reads

	    By default, is set to 1, that is, VxFS detects only sequential patterns.

	    detects patterns on a per-thread basis, up to a maximum of The default number of threads is 5, however, you  can  change  the  default
	    value by setting the parameter in the system configuration file,

       The number of parallel read
	    requests  of  size	to  have  outstanding at one time.  The file system uses the product of and to determine its read ahead size.  The
	    default value for is 1.

       The preferred read request size.  The
	    file system uses this in conjunction with the value to determine how much data to read ahead.  The default value is 64K.

       The number of parallel write
	    requests of size to have outstanding at one time.  The file system uses the product of and to determine when to  do  flush	behind	on
	    writes.  The default value for is 1.

       The preferred write request size.  The
	    file system uses this in conjunction with the value to determine how to do flush behind on writes.	The default value is 64K.

       When data is written to a file through buffered writes,
	    the  file  system  updates	only  the  in-memory image of the file, creating what are referred to as dirty buffers.  Dirty buffers are
	    cleaned when the the file system later writes the data in these buffers to disk.  (Note that data can be lost if  the  system  crashes
	    before dirty buffers are written to disk.)

	    Newer  model computer systems typically have more memory.  The more physical memory a system has, the more dirty buffers the file sys-
	    tem can generate before having to write the buffers to disk to free up memory.  So more dirty buffers can potentially lead	to  longer
	    return times for operations that write dirty buffers to disk such as and If your system has a combination of a slow storage device and
	    a large amount of memory, the sync operations may take long enough to complete that it gives the appearance of a hung system.

	    If your system is exhibiting this behavior, you can change the value of lets you lower the number of dirty buffers per file  that  the
	    file  system  will generate before writing them to disk.  After the number of dirty buffers for a file reaches the threshold, the file
	    system starts flushing buffers to disk even if free memory is still available.  Depending on the speed of  the  storage  device,  user
	    write performance may suffer, but the number of dirty buffers is limited, so sync operations will complete much faster.

	    The  default value of is zero.  The default value places no limit on the number of dirty buffers per file.	This typically generates a
	    large number of dirty buffers, but maintains fast writes.  If is non-zero, VxFS limits the number of dirty buffers per file to buffers
	    In	some  cases, may delay write requests.	For example, lowering the value of may increase the file disk queue to the value, delaying
	    user writes until the disk queue decreases.  So unless the system has a combination of large physical memory and slow storage devices,
	    it is advisable not to change the value of

FILES

       VxFS file system tuning parameters table.

SEE ALSO

       mkfs_vxfs(1M), mount(1M), mount_vxfs(1M), sync(2), tunefstab(4), vxfsio(7).

																      vxtunefs(1M)