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

Buffered and Direct I/O

VxFS responds with read-ahead for sequential read I/O. This results in buffered

I/O. The data is prefetched and retained in buffers for the application. This is the

default VxFS behavior.

On the other hand, direct I/O does not buffer the data when the I/O to the

underlying device is completed. This saves system resources like memory and

CPU usage. Direct I/O is possible only when alignment and sizing criteria are

satisfied.

See “Direct I/O requirements” on page 70.

Each of the supported platforms has either a page cache or its own buffer cache.

These caches are commonly known as the file system caches.

Direct I/O does not use these caches. The memory used for direct I/O is discarded

after the I/O is complete,

Direct I/O

Direct I/O is an unbuffered form of I/O. If the VX_DIRECT advisory is set, the user

is requesting direct data transfer between the disk and the user-supplied buffer

for reads and writes. This bypasses the kernel buffering of data, and reduces the

CPU overhead associated with I/O by eliminating the data copy between the kernel

buffer and the user's buffer. This also avoids taking up space in the page cache

that might be better used for something else. The direct I/O feature can provide

significant performance gains for some applications.

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

basis.

JFS (base VxFS) includes direct I/O without a license.

Direct I/O requirements

For an I/O operation to be performed as direct I/O, it must meet certain alignment

criteria. The alignment constraints are usually determined by the disk driver, the

disk controller, and the system memory management hardware and software.

The requirements for direct I/O are as follows:

■ The starting file offset must be aligned to a 1024-byte boundary.

■ The ending file offset must be aligned to a 1024-byte boundary, or the length

must be a multiple of 1024 bytes.

■ The memory buffer must start on an 4-byte boundary.

Veritas File System I/O

Buffered and Direct I/O