Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

About I/O statistics
The output of the qiostat command is the primary source of information to use
in deciding whether to enable or disable Cached Quick I/O on specific files.
Statistics are printed in two lines per object.
The second line of information is defined as follows:
■ CREAD is the number of reads from the VxFS cache (or total number of reads
to Quick I/O files with cache advisory on)
■ PREAD is the number of reads going to the disk for Quick I/O files with the cache
advisory on
■ HIT RATIO is displayed as a percentage and is the number of CREADS minus
the number of PREADS times 100 divided by the total number of CREADS. The
formula looks like this:
(CREADs - PREADs) * 100/ CREADs
The qiostat -l command output looks similar to the following:
/db01/tbs1_cont001 6 1 21 4 10.0 0.0
6 6 0.0
/db01/tbs2_cont001 62552 38498 250213 153992 21.9 0.4
62567 49060 21.6
/db01/tbs2_cont002 62552 38498 250213 153992 21.9 0.4
62567 49060 21.6
Analyze the output to find out where the cache-hit ratio is above a given threshold.
A cache-hit ratio above 20 percent on a file for a given application may be sufficient
to justify caching on that file. For systems with larger loads, the acceptable ratio
may be 30 percent or above. Cache-hit-ratio thresholds vary according to the
database type and load.
Using the sample output above as an example, the file /db01/tbs1_cont001 does
not benefit from the caching because the cache-hit ratio is zero. In addition, the
file receives very little I/O during the sampling duration.
However, the files /db01/tbs2_* have a cache-hit ratio of 21.6 percent. If you
have determined that, for your system and load, this figure is above the acceptable
threshold, it means the database can benefit from caching. Also, study the numbers
reported for the read and write operations. When you compare the number of
reads and writes for the /db01/tbs2_* files, you see that the number of reads is
roughly twice the number of writes. You can achieve the greatest performance
83Improving DB2 database performance with Veritas Cached Quick I/O
Determining candidates for Cached Quick I/O