VERITAS File System 4.1 Administrator's Guide

VxFS Performance: Creating, Mounting, and Tuning File Systems
Choosing mount Command Options
Chapter 2 45
convosync
NOTE Use of the convosync=dsync option violates POSIX guarantees for
synchronous I/O.
The convosync (convert osync) mode has five suboptions:
• convosync=closesync
• convosync=delay.
• convosync=direct
• convosync=dsync
• convosync=unbuffered
The convosync=closesync mode converts synchronous and data synchronous writes to
non-synchronous writes and flushes the changes to the file to disk when the file is closed.
The convosync=delay mode causes synchronous and data synchronous writes to be delayed
rather than to take effect immediately. No special action is performed when closing a file. This
option effectively cancels any data integrity guarantees normally provided by opening a file
with O_SYNC. See the open (2), fcntl (2), and vxfsio (7) manual pages for more information on
O_SYNC.
CAUTION Be very careful when using the convosync=closesync or convosync=delay
mode because they actually change synchronous I/O into non-synchronous I/O.
This may cause applications that use synchronous I/O for data reliability to fail
if the system crashes and synchronously written data is lost.
The convosync=direct and convosync=unbuffered mode convert synchronous and data
synchronous reads and writes to direct reads and writes.
The convosync=dsync mode converts synchronous writes to data synchronous writes.
As with closesync, the direct, unbuffered, and dsync modes flush changes to the file to
disk when it is closed. These modes can be used to speed up applications that use synchronous
I/O. Many applications that are concerned with data integrity specify the O_SYNC fcntl in
order to write the file data synchronously. However, this has the undesirable side effect of
updating inode times and therefore slowing down performance. The convosync=dsync,
convosync=unbuffered, and convosync=direct modes alleviate this problem by allowing
applications to take advantage of synchronous writes without modifying inode times as well.