VERITAS Storage Foundation 4.1 Oracle Administrator's Guide

Determining Candidates for Cached Quick I/O Prerelease 8 September 2005, 8:54am
118 VERITAS Storage Foundation for Oracle Administrator’s Guide
Based on these two factors, /db01/stk.dbf is a prime candidate for Cached Quick I/O. For
more information on enabling and disabling Cached Quick I/O at the file level, see “Enabling and
Disabling Cached Quick I/O for Individual Files” on page 120.
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 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.