hpux man page for fs_async

fs_async(5)							File Formats Manual						       fs_async(5)

NAME
       fs_async - enables write calls to return before write operation is complete

VALUES
   Failsafe
   Default
   Allowed values
       The allowed values are:
       (use synchronous disk writes only), or
       (allow asynchronous disk writes).

       Specify a positive integer value, or

DESCRIPTION
       specifies whether or not asynchronous writing of file system data structures to disk is allowed.

       Synchronous  writes  to disk make it easier to restore file system integrity if a system crash occurs while file system data structures are
       being updated on the file system.

       If asynchronous writes are selected, HP-UX file system semantics for NFS cluster environments are preserved.   In  addition,  files  opened
       using  with  the flag (synchronous writing) will continue to be written synchronously when the asynchronous-writes feature has been config-
       ured into the kernel.

       Asynchronous writes to disk can improve file system performance significantly.  However, asynchronous writes can  leave	file  system  data
       structures  in an inconsistent state in the event of a system crash.  For more information about when to select synchronous or asynchronous
       writing, see the following tutorial.

   Restrictions on Changing
       This tunable is static.	Any changes to the value of this tunable will require system reboot before taking effect.

   Tutorial: What are Synchronous and Asynchronous Writes?
       If a file is open for writing and data is being written to a file, the data is accumulated in buffers and  periodically	written  to  disk.
       When  an  end-of-file  condition  occurs  and the file is to be closed, any remaining buffer contents are written to the disk, the inode is
       updated with file size and block pointer information, and the file system's list of free disk blocks is updated.  To ensure maximum protec-
       tion of file system integrity, these operations are handled in a specific sequence that minimizes the risk of file system corruption on the
       disk if a system crash or power failure occurs while writing to the disk.  This sequential update process is called

       HP-UX file systems store free space lists, blocks, inodes, and other file components in	random	and  widely  separate  locations  on  disk
       devices.  This means that writing file information blocks in a particular sequence requires additional time to move to the desired location
       on the disk before performing the write operation.  If a power failure or system crash occurs during this sequence, one or more blocks  may
       not be properly updated, leaving a potentially inconsistent file system.  The command is used to repair such inconsistencies.

       Asynchronous writing as it relates to the kernel parameter allows the system to update file system information on the disk in a more conve-
       nient (hence faster) sequence rather than in a more secure (safer but slower) sequence,	thus  reducing	search	and  move  delays  between
       writes.	 However, if a system crash occurs while these operations are being performed, the risk of an inconsistent file system that cannot
       be automatically repaired by is significantly greater than with synchronous writes.  If only synchronous writing is used,  all  updates	to
       directories,  file inodes, free space lists, and so on are handled in a sequence that is known to If a crash occurs while updating any disk
       block in the sequence, can readily determine where the crash occurred and repair the missing update information,  probably  without  assis-
       tance from the system administrator.

       If is set to allow asynchronous writes and a crash occurs, does not know what sequence was used, and thus will probably require interactive
       assistance from the administrator while fixing inconsistent file system information, repairing directory and  inode  entries,  and  so  on.
       Waiting	for  synchronous writing and updating of disk blocks when closing files after writing to them degrades the performance of programs
       and applications that require frequent file and directory write and close operations.  Allowing asynchronous writing significantly  reduces
       those  delays,  producing  a corresponding improvement in performance.  However, when applications are CPU intensive with relatively little
       disk I/O, performance improvements are much lower.  Asynchronous writing is advisable for improving system performance if:

	      o  Risk of power failure is low (very dependable power source and/or uninterruptible power sources).

	      o  Precautions have been taken to enhance data security (sophisticated file system backup or redundancy  strategies),  or  potential
		 loss of data due to a system crash is less important than system performance.

	      o  User applications require frequent opening, writing, and closing of disk files and directories.

	      o  Elimination of synchronous writing would improve system performance sufficiently to offset any associated risks.

       To enable asynchronous writing, set the kernel parameter to instead of the default value of

WARNINGS
       All  HP-UX kernel tunable parameters are release specific.  This parameter may be removed or have its meaning changed in future releases of
       HP-UX.

       Installation of optional kernel software, from HP or other vendors, may cause changes to tunable  parameter  values.   After  installation,
       some  tunable  parameters may no longer be at the default or recommended values.  For information about the effects of installation on tun-
       able values, consult the documentation for the kernel software being installed.	For information about optional kernel  software  that  was
       factory installed on your system, see at

AUTHOR
       was developed by HP.

SEE ALSO
       fsck(1M), kctune(1M), sam(1M), gettune(2), open(2), settune(2).

							     Tunable Kernel Parameters						       fs_async(5)