Drive report
The drive report contains performace-related information for physical drives.
When you suspect a disk I/O performance problem, use the iostat command. To avoid the information about the TTY and CPU statistics, use the -d option. In addition, the disk statistics can be limited to the important disks by specifying the disk names.
Remember that the first set of data represents all activity since system startup.
- Disks:
- Shows the names of the physical volumes. They are either hdisk or cd followed by a number. If physical volume names are specified with the iostat command, only those names specified are displayed.
- % tm_act
- Indicates the percentage of time that the physical disk was active (bandwidth utilization for the drive) or, in other words, the total time disk requests are outstanding. A drive is active during data transfer and command processing, such as seeking to a new location. The "disk active time" percentage is directly proportional to resource contention and inversely proportional to performance. As disk use increases, performance decreases and response time increases. In general, when the utilization exceeds 70 percent, processes are waiting longer than necessary for I/O to complete because most UNIX processes block (or sleep) while waiting for their I/O requests to complete. Look for busy versus idle drives. Moving data from busy to idle drives can help alleviate a disk bottleneck. Paging to and from disk will contribute to the I/O load.
- Kbps
- Indicates the amount of data transferred (read or written) to the drive in KB per second. This is the sum of Kb_read plus Kb_wrtn, divided by the seconds in the reporting interval.
- tps
- Indicates the number of transfers per second that were issued to the physical disk. A transfer is an I/O request through the device driver level to the physical disk. Multiple logical requests can be combined into a single I/O request to the disk. A transfer is of indeterminate size.
- Kb_read
- Reports the total data (in KB) read from the physical volume during the measured interval.
- Kb_wrtn
- Shows the amount of data (in KB) written to the physical volume during the measured interval.
Taken alone, there is no unacceptable value for any of the above fields because statistics are too closely related to application characteristics, system configuration, and type of physical disk drives and adapters. Therefore, when you are evaluating data, look for patterns and relationships. The most common relationship is between disk utilization (%tm_act) and data transfer rate (tps).
To draw any valid conclusions from this data, you have to understand the application's disk data access patterns such as sequential, random, or combination, as well as the type of physical disk drives and adapters on the system. For example, if an application reads/writes sequentially, you should expect a high disk transfer rate (Kbps) when you have a high disk busy rate (%tm_act). Columns Kb_read and Kb_wrtn can confirm an understanding of an application's read/write behavior. However, these columns provide no information on the data access patterns.
Generally you do not need to be concerned about a high disk busy rate (%tm_act) as long as the disk transfer rate (Kbps) is also high. However, if you get a high disk busy rate and a low disk transfer rate, you may have a fragmented logical volume, file system, or individual file.
Discussions of disk, logical volume and file system performance sometimes lead to the conclusion that the more drives you have on your system, the better the disk I/O performance. This is not always true because there is a limit to the amount of data that can be handled by a disk adapter. The disk adapter can also become a bottleneck. If all your disk drives are on one disk adapter, and your hot file systems are on separate physical volumes, you might benefit from using multiple disk adapters. Performance improvement will depend on the type of access.
To see if a particular adapter is saturated, use the iostat command and add up all the Kbps amounts for the disks attached to a particular disk adapter. For maximum aggregate performance, the total of the transfer rates (Kbps) must be below the disk adapter throughput rating. In most cases, use 70 percent of the throughput rate. In the AIX operating system, the -a or -A option will display this information.