VERITAS File System 4.1 Administrator's Guide

Quick I/O for Databases
Cached Quick I/O For Databases
Chapter 11 173
Cached Quick I/O For Databases
32-bit applications (such as 32-bit databases) can use a maximum of only 4 GB of memory
because of the 32-bit address limitation. The Cached Quick I/O feature improves database
performance on machines with sufficient memory by also using the file system cache to store
data.
For read operations through the Quick I/O interface, data is cached in the system buffer
cache, so subsequent reads of the same data can access this cached copy and avoid doing disk
I/O. To maintain the correct data in its buffer for write operations, Cached Quick I/O keeps
the buffer cache in sync with the data written to disk.
With 64-bit applications, for which limited memory is not a critical problem, using the
filesystem cache still provides performance benefits by using the read-ahead functionality.
Because of the read-ahead functionality, sequential table scans will benefit the most from
using Cached Quick I/O by significantly reducing the query response time.
To use this feature, set the qio_cache_enable system parameter with the vxtunefs utility,
and use the qioadmin command to turn the per-file cache advisory on or off. See the
vxtunefs(1M) and qioadmin(1) man pages for more information.
Enabling Cached Quick I/O
You can enable caching for Quick I/O files online when the database is running as follows:
1. Set the qio_cache_enable parameter of vxtunefs to enable caching on a file system.
2. Enable the Cached Quick I/O feature for specific files using the qioadmin command.
NOTE You must enable Quick I/O on the file system to enable Cached Quick I/O.
Enabling Cached Quick I/O for File Systems
Caching is initially disabled on a file system. You enable Cached Quick I/O for a file system by
setting the qio_cache_enable option of the vxtunefs command after the file system is
mounted. For example, to enable Cached Quick I/O for the file system /database01, enter:
# vxtunefs -s -o qio_cache_enable=1 /database01
where /database01 is a VxFS file system containing the Quick I/O files.