Veritas Storage Foundation™ for Oracle 5.0.1 Administrator's Guide

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/system.dbf 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 file /db01/stk.dbf has 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/stk.dbf file, you see that the number of reads is
roughly twice the number of writes. You can achieve the greatest performance
gains with Cached Quick I/O when using it for files that have higher read than
write activity.
Based on these two factors, /db01/stk.dbf is a prime candidate for Cached Quick
I/O.
See “Enabling and disabling Cached Quick I/O for individual files” on page 111.
Effects of read-aheads on I/O statistics
The number of CREADs in the qiostat output is the total number of reads
performed, including Cached Quick I/O, and the number of PREADs is the number
of physical reads. The difference between CREADs and PREADs (CREADS - PREADS) is
the number of reads satisfied from the data in the file system cache. Thus, you
expect that the number of PREADs would always be equal to or lower than the
number of CREADs.
However, the PREADs counter also increases when the file system performs
read-aheads. These read-aheads occur when the file system detects sequential
reads. In isolated cases where cache hits are extremely low, the output from
qiostat could show that the number of CREADs is lower than the number of PREADs.
The cache-hit ratio calculated against these CREAD/PREAD values is misleading
when used to determine whether Cached Quick I/O should be enabled or disabled.
Under these circumstances, you can make a more accurate decision based on a
collective set of statistics by gathering multiple sets of data points. Consequently,
you might want to enable Cached Quick I/O for all the data files in a given
tablespace, even if just one of the files exhibited a high cache-hit ratio.
Using Veritas Cached Quick I/O
Determining candidates for Cached Quick I/O
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