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You have a choice of several excellent tools to measure I/O performance. Several operating system utilities are easy to use and give you information about how busy your disks are. Iostat and sar are two of the popular operating system utilities that measure disk performance. I explained how to use both these tools in 3.

From the sar output, you can figure out whether you re using the storage subsystem heavily. If the number of waits is higher than the number of CPUs, or if the service times are high (say, greater than 20 milliseconds), then your system is facing contention at the I/O level. One of the most useful pieces of information you can get is by using the sar d command to find out if you re using any of your disks excessively compared to other disks in the system. Once you identify such hot spots, you can move the datafiles to less busy drives, thereby spreading the load more evenly. The following is the output of a sar d command that shows extremely high queue values. Even at peak levels, the avque column value should be less than 2. Here, it is 61.4. Obviously, something is happening on the file system named c2t6d0 that s showing up as a high queue value: $ sar -d 10 5 HP-UX finance1 B.11.00 A 9000/800 17:27:13 device %busy avque 17:27:23 c2t6d0 100 61.40 c5t6d0 20.38 0.50 c2t6d0 100 61.40 c5t6d0 18.28 0.50 c0t1d0 0.10 0.50 . . . $ 07/03/08 r+w/s blks/s 37 245 28 208 38 273 27 233 4 33

You can obtain an idea about the I/O distribution in your system by using the query in Listing 20-13.

Forces When a blind interaction is underway, the caller has no way of knowing how far along the callee is or how soon the callee will finish. The caller can only monitor the amount of time the callee is taking to respond, or other side effects of the interaction. Certain conditions may require the caller to abort a command in progress.

Listing 20-13. Determining I/O Distribution in the Database SQL> 2 3 4 5 6 7 8 9 10 11 12* SELECT d.name, f.phyrds reads, f.phywrts wrts, (f.readtim / decode(f.phyrds,0,-1,f.phyrds)) readtime, (f.writetim / decode(f.phywrts,0,-1,phywrts)) writetime FROM v$datafile d, v$filestat f WHERE d.file# = f.file# ORDER BY d.name; WRTS ---9 7 READTIME ---------.608695652 .277777778 WRITETIME ---------.222222222 0

NAME READS ----------------------------- ----/pa01/oradata/pa/lol_i_17.dbf 23 /pa01/oradata/pa/lol_i_18.dbf 18 . . . SQL>

Blind interactions have the drawback that the caller has no way to determine how long a command will take, or even if the command will succeed, except perhaps for previous experience with that command. Even so, a system whose computing resources vary at run time may be difficult to predict in terms of how quickly a command will be executed. There are many situations in which a system may need to attempt a command. Should the command fail to complete within a certain time limit, the system may opt to abort the command and change the course of action. In another scenario, a caller may initiate concurrent commands in multiple callees through asynchronous blind interactions. The caller might want to execute the commands as a single transaction. Should any of the commands fail, all other commands would have to be cancelled. A subsequent rollback operation might be necessary to restore the system to its original state before the failure. The rollback operation itself is not part of the interruptible interaction pattern. Figure 9-16 shows an interruptible blind interaction.

Excessive reads and writes on some disks indicate that there might be disk contention in your I/O system.

If there s severe I/O contention in your system, you can undertake some of the following steps, depending on your present database configuration: Increase the number of disks in the storage system. Separate the database and the redo log files. For a large table, use partitions to reduce I/O. Stripe the data either manually or by using a RAID disk-striping system. Invest in cutting-edge technology, such as file caching, to avoid I/O bottlenecks. Consider using the new Automatic Storage Management system, which is discussed in 17.