About I/O Types

This section provide information about paged I/O, direct I/O, and switching between the I/Os.

Paged I/O

When paged I/O is used, user data is cached in virtual memory pages, and the kernel writes the data to disk. The standard Oracle Solaris OS interfaces manage paged I/O. Paged I/O (also called buffered or cached I/O) is selected by default.

Direct I/O

Direct I/O is a process by which data is transferred directly between the user's buffer and the disk. This means that much less time is spent in the system. For performance purposes, specify direct I/O only for large, block-aligned, sequential I/O.

The setfa command and the sam_setfa library routine both have a-D option that sets the direct I/O attribute for a file or directory. If applied to a directory, files and directories created in that directory inherit the direct I/O attribute. After the-D option is set, the file uses direct I/O.

You can also select direct I/O for a file by using the Oracle Solaris OS directio function call. If you use the function call to enable direct I/O, the setting lasts only while the file is active.

To enable direct I/O on a file system basis, do one of the following:

• Specify the-o forcedirectio option with the mount command.

• Put the forcedirectio keyword in the mount option column of the /etc/vfstab file, or use it as a directive in the samfs.cmd file.

For more information, see the setfa(1), sam_setfa(3), samfs.cmd(4), directio(3C), and mount_samfs(1M) man pages in Sun QFS and Sun Storage Archive Manager 5.3 Reference Manual.

I/O Switching

By default, paged I/O is performed and I/O switching is disabled. However, the Sun QFS file systems support automatic I/O switching, a process by which a site-defined amount of paged I/O occurs before the system switches automatically to direct I/O.

I/O switching should reduce page cache usage on large I/O operations. To enable I/O switching, use samu(1M), or use the dio_wr_consec and dio_rd_consec parameters as directives in the samfs.cmd file or as options with the mount command.

For more information about these options, see mount_samfs(1M) in Sun QFS and Sun Storage Archive Manager 5.3 Reference Manual or samfs.cmd(4) in Sun QFS and Sun Storage Archive Manager 5.3 Reference Manual.