fs_async(5)fs_async(5)NAMEfs_async - enables write calls to return before write operation is com‐
plete
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 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
WARNINGS
All HP-UX kernel tunable parameters are release specific. This parame‐
ter 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 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
AUTHOR
was developed by HP.
SEE ALSOfsck(1M), kctune(1M), sam(1M), gettune(2), open(2), settune(2).
Tunable Kernel Parameters fs_async(5)
