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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 26.32Average 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