Beginning Oracle Database 11g Administration From Novice to Professional

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CHAPTER 16 INSTANCE TUNING
The most important section of the Statspack report is the “Top 5 Timed Events” section.
It is preceded by details concerning the workload processed by the database. In
Listing 16-2, you can see that the DB time was 1,589.75 minutes during an elapsed interval
of 60.55 minutes. DB time is the sum of the response times of all database calls made
by the connected sessions. The reason it is so much larger than the clock time is because
many database sessions were simultaneously active. Note that actual CPU usage was only
7.88 minutes, less than one half of 1 percent of DB time. The rest of the time was spent
waiting for overhead activities such as reading data from disk (
operations) and writing redo information to the online redo logs ( operations).
Each operation retrieves one 8 KB block of data from disk.
The average time for each operation was 239 milliseconds, which is totally unacceptable
and indicates that the disks were the bottleneck; it should not take more than 5 milliseconds
to retrieve a block of data. The average time for each operation was
426 milliseconds—more than four-tenths of a second—another indication that the disks
are the bottleneck.
The best way to tune this database would be to get faster disks; another alternative
would be to increase the size of the buffer cache so that less data has to be read from disk.