VERITAS Storage Foundation 4.1 Oracle Administrator's Guide

Understanding Quick I/O Prerelease 8 September 2005, 8:54am
84 VERITAS Storage Foundation for Oracle Administrator’s Guide
Understanding Quick I/O
How Quick I/O Works
VERITAS Quick I/O supports direct I/O and allows databases to access regular files on a VxFS file
system as raw character devices.
The benefits of using Quick I/O for Oracle databases are:
◆ Improved performance and processing throughput by having Quick I/O files act as raw devices
◆ Ability to manage Quick I/O files as regular files, which simplifies administrative tasks such as
allocating, moving, copying, resizing, and backing up datafiles
Note that VERITAS recommends using Oracle Disk Manager. If you would like to migrate from
Quick I/O to Oracle Disk Manager, see “Converting Quick I/O Files to Oracle Disk Manager Files”
on page 133 for more information.
How Quick I/O Improves Database Performance
Quick I/O’s ability to access regular files as raw devices improves database performance by:
◆ Supporting direct I/O
◆ Avoiding kernel write locks on database files
◆ Avoiding double buffering
Supporting
is a form of I/O that performs non-blocking system level reads and writes, allowing the system to
handle multiple I/O requests simultaneously. Operating systems such as HP-UX 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 runs on file systems. Lack of asynchronous I/O significantly degrades
performance. Quick I/O lets the database server take advantage of kernel-supported asynchronous
I/O on file system files accessed using the Quick I/O interface.
Supporting Direct I/O
I/O on files using 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 extra copying.