Tuning I/O resources

Viewing disk and other block I/O activity

The activity of block devices installed on the system, including floptical, floppy and hard disk drives, CD-ROM and SCSI tape drives, can be examined using sar -d (or mpsar -d for SMP). This example shows the activity for a single SCSI disk:

   23:59:44  device  %busy    avque    r+w/s   blks/s   avwait   avserv
   23:59:49  Sdsk-0  99.42     4.18    39.39   166.28    80.26    25.24
   23:59:54  Sdsk-0 100.00     4.18    38.73   163.64    82.35    25.87
   23:59:59  Sdsk-0 100.00     3.98    38.07   171.95    78.32    26.32

Average Sdsk-0 99.89 4.12 38.78 167.21 80.32 25.76

device shows the name of the device whose activity is being reported. In this example, the device is the first SCSI disk in the system.

%busy indicates the percentage of time that the system was transferring data to and from the device.

avque indicates the average number of requests pending on the device including any on the device itself. This number is usually greater than the number of processes waiting to access the device if scatter-gather read ahead is being performed on behalf of a filesystem.

avwait represents the average time in milliseconds that the request waits in the driver before being sent to the device.

avserv represents the average time in milliseconds that it takes a request to complete. The length of time is calculated from the time that the request was sent to the device to the moment that the device signals that it has completed the request. Note that avserv values vary considerably according to the type of disk and any caching on the disk controller.

r+w/s is the number of read and write transfers from and to the disk, and blks/s is the number of 512-byte blocks transferred per second. These two values can be used to calculate the average size of data transfers using the formula:

Average size of data transfer = blks/s / r+w/s

Next topic: Identifying disk I/O-bound systems
Previous topic: The performance characteristics of a hard disk

© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003