HP-UX Reference (11i v3 07/02) - 5 Miscellaneous Topics (vol 9)

f
fs_async(5) fs_async(5)
(Tunable Kernel Parameters)
NAME
fs_async - enables write calls to return before write operation is complete
VALUES
Failsafe
0
Default
0
Allowed values
The allowed values are:
0 (use synchronous disk writes only), or
1 (allow asynchronous disk writes).
Specify a positive integer value, 0 or
1.
DESCRIPTION
fs_async 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 open() with the 0_SYNC flag (synchronous writing) will con-
tinue to be written synchronously when the asynchronous-writes feature has been configured into the ker-
nel.
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 periodi-
cally 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 synchronous writing.
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 fsck command is used to repair such incon-
sistencies.
Asynchronous writing as it relates to the fs_async kernel parameter allows the system to update file
system information on the disk in a more convenient (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 fsck is significantly greater than with synchronous writes.
Consequences of a Crash
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 fsck. If a crash occurs while updating any disk block in the
sequence, fsck can readily determine where the crash occurred and repair the missing update informa-
tion, probably without assistance from the system administrator.
If fs_async is set to allow asynchronous writes and a crash occurs, fsck does not know what sequence
was used, and thus will probably require interactive assistance from the administrator while fixing
176 Hewlett-Packard Company − 1 − HP-UX 11i Version 3: February 2007