VERITAS File SystemÖ 3.5 (HP OnlineJFS/JFS3.5) AdministratorÆs Guide (December 2002)

Chapter 2

VxFS Performance: Creating, Mounting, and Tuning File Systems

Choosing mount Command Options

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.

CAUTION Before using convosync=dsync, convosync=unbuffered, or convosync=direct, make sure

that all applications that use the file system do not require synchronous inode time updates for

O_SYNC writes.