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

gains with Cached Quick I/O when using it for files that have higher read than
write activity.
Based on these two factors, /db01/tbs2_cont001 and /db01/tbs2_cont002 are
prime candidates for Cached Quick I/O.
See “Enabling and disabling Cached Quick I/O for individual files” on page 85.
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 container files that contain
a particular table, across multiple tablespaces used by a given database, even if
the containers in just one of the tablespaces exhibited a high cache hit ratio. In
general, we expect all containers in a tablespace to have approximately the same
cache hit ratio.
Other tools for analysis
While the output of the qiostat command is the primary source of information
to use in deciding whether to enable Cached Quick I/O on specific files, we also
recommend using other tools in conjunction with qiostat. For example,
benchmarking software that measures database throughput is also helpful. If a
benchmark test in which Cached Quick I/O was enabled for a certain set of data
files resulted in improved performance, you can also use those results as the basis
for enabling Cached Quick I/O.
Improving DB2 database performance with Veritas Cached Quick I/O
Determining candidates for Cached Quick I/O
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