fs_async man page on HP-UX

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fs_async(5)							   fs_async(5)

       fs_async - enables write calls to return before write operation is com‐

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

       Specify a positive integer value, or

       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  configured
       into the kernel.

       Asynchronous writes to disk can improve file system performance signif‐
       icantly.	 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 protection 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 particu‐
       lar 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 per‐
       formed, the risk of an inconsistent file system that cannot be automat‐
       ically repaired by  is  significantly  greater  than  with  synchronous
       writes.	 If  only synchronous writing is used, all updates to directo‐
       ries, 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 assistance
       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 applica‐
       tions that require frequent file and directory write and	 close	opera‐
       tions.	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:

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

	      ·	 Precautions have been taken to enhance data security (sophis‐
		 ticated  file	system	backup	or  redundancy strategies), or
		 potential loss of data due to a system crash is  less	impor‐
		 tant than system performance.

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

	      ·	 Elimination of synchronous writing would improve system  per‐
		 formance sufficiently to offset any associated risks.

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

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

       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  val‐
       ues.  For information about the effects of installation on tunable val‐
       ues, consult the documentation for the kernel software being installed.
       For  information	 about	optional  kernel  software  that  was  factory
       installed on your system, see at

       was developed by HP.

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

			   Tunable Kernel Parameters		   fs_async(5)

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