Veritas File System 5.1 SP1 Administrator"s Guide (5900-1499, April 2011)

and, if necessary, an increased file size. In data synchronous I/O, the data is

transferred to disk synchronously before the write returns to the user. If the file

is not extended by the write, the times are updated in memory, and the call returns

to the user. If the file is extended by the operation, the inode is written before the

write returns.

The direct I/O and VX_DSYNC advisories are maintained on a per-file-descriptor

basis.

Data synchronous I/O vs. synchronous I/O

Like direct I/O, the data synchronous I/O feature can provide significant

application performance gains. Because data synchronous I/O maintains the same

data integrity as synchronous I/O, it can be used in many applications that

currently use synchronous I/O. If the data synchronous I/O does not allocate

storage or extend the file, the inode is not immediately written. The data

synchronous I/O does not have any alignment constraints, so applications that

find it difficult to meet the alignment constraints of direct I/O should use data

synchronous I/O.

If the file is being extended or storage is allocated, data synchronous I/O must

write the inode change before returning to the application. This case eliminates

the performance advantage of data synchronous I/O.

Concurrent I/O

Concurrent I/O (VX_CONCURRENT) allows multiple processes to read from or write

to the same file without blocking other read(2) or write(2) calls. POSIX semantics

requires read and write calls to be serialized on a file with other read and write

calls. With POSIX semantics, a read call either reads the data before or after the

write call occurred. With the VX_CONCURRENT advisory set, the read and write

operations are not serialized as in the case of a character device. This advisory is

generally used by applications that require high performance for accessing data

and do not perform overlapping writes to the same file. It is the responsibility of

the application or the running threads to coordinate the write activities to the

same file when using Concurrent I/O.

Concurrent I/O can be enabled in the following ways:

■ By specifying the VX_CONCURRENT advisory flag for the file descriptor in the

VX_SETCACHE ioctl command. Only the read(2) and write(2) calls occurring

through this file descriptor use concurrent I/O. The read and write operations

occurring through other file descriptors for the same file will still follow the

POSIX semantics.

Veritas File System I/O

Concurrent I/O