VERITAS File System 4.1 Administrator's Guide

Quick I/O for Databases
Quick I/O Functionality and Performance
Chapter 11 161
Quick I/O Functionality and Performance
Many database administrators (DBAs) create databases on file systems because it makes
common administrative tasks (such as moving, copying, and backup) much simpler. However,
putting databases on file systems significantly reduces database performance. By using
VERITAS Quick I/O, you can retain the advantages of having databases on file systems
without performance degradation.
Quick I/O uses a special naming convention to allow database applications to access regular
files as raw character devices. This provides higher database performance in the following
ways:
• Supporting kernel asynchronous I/O
• Supporting direct I/O
• Avoiding kernel write locks
• Avoiding double buffering
Supporting Kernel Asynchronous I/O
Some operating systems provide kernel support for asynchronous I/O on raw devices, but not
on regular files. As a result, even if the database server is capable of using asynchronous I/O,
it cannot issue asynchronous I/O requests when the database is built on a file system. Lack of
asynchronous I/O significantly degrades performance. Quick I/O lets the database server take
advantage of kernel supported asynchronous I/O (through the asyncdsk or Posix AIO
interface) on file system files accessed via the Quick I/O interface by providing a character
device node that is treated by the OS as a raw device.
Supporting Direct I/O
I/O on files using the read() and write() system calls typically results in data being copied
twice: once between user and kernel space, and later between kernel space and disk. In
contrast, I/O on raw devices is direct. That is, data is copied directly between user space and
disk, saving one level of copying. As with I/O on raw devices, Quick I/O avoids the extra
copying.