Chapter 3. Bandwidth and Processing Power

37

names, it is possible to use this report to determine if, for example,

/home

is experiencing an excessive

workload.

Actually, each line output from

iostat  x

is longer and contains more information than this; here is

the remainder of each line (with the device column added for easier reading):

Device:

avgqu sz

await

svctm

%util

/dev/sda

0.24

20.86

3.80

0.43

/dev/sda1

0.00

141.18 122.73

0.03

/dev/sda2

0.00

6.00

6.00

0.00

/dev/sda3

0.12

12.84

2.68

0.24

/dev/sda4

0.11

57.47

8.94

0.17

In this example, it is interesting to note that

/dev/sda2

is the system swap partition; it is obvious

from the many fields reading

0.00

for this partition that swapping is not a problem on this system.

Another interesting point to note is

/dev/sda1

. The statistics for this partition are unusual; the over 

all activity seems low, but why are the average I/O request size (the

avgrq sz

field), average wait

time (the

await

field), and the average service time (the

svctm

field) so much larger than the other

partitions? The answer is that this partition contains the

/boot/

directory, which is where the kernel

and initial ramdisk are stored. When the system boots, the read I/Os (notice that only the

rsec/s

and

rkB/s

fields are non zero; no writing is done here on a regular basis) used during the boot process are

for large numbers of blocks, resulting in the relatively long wait and service times

iostat

displays.

It is possible to use

sar

for a longer term view of I/O statistics; for example,

sar  b

displays a

general I/O report:

Linux 2.4.18 18.8.0smp (raptor.example.com)

12/15/2002

12:00:00 AM

tps

rtps

wtps

bread/s

bwrtn/s

12:10:00 AM

0.51

0.01

0.50

0.25

14.32

12:20:01 AM

0.48

0.00

0.48

0.00

13.32

...

06:00:02 PM

1.24

0.00

1.24

0.01

36.23

Average:

1.11

0.31

0.80

68.14

34.79

Here, like

iostat

's initial display, the statistics are grouped for all block devices.

Another I/O related report is produced using

sar  d

:

Linux 2.4.18 18.8.0smp (raptor.example.com)

12/15/2002

12:00:00 AM

DEV

tps

sect/s

12:10:00 AM

dev8 0

0.51

14.57

12:10:00 AM

dev8 1

0.00

0.00

12:20:01 AM

dev8 0

0.48

13.32

12:20:01 AM

dev8 1

0.00

0.00

...

06:00:02 PM

dev8 0

1.24

36.25

06:00:02 PM

dev8 1

0.00

0.00

Average:

dev8 0

1.11

102.93

Average:

dev8 1

0.00

0.00

This report provides a per device view, but with little detail.

As we can see, while there are no explicit statistics that show bandwidth utilization for a given bus or

datapath, we can at least see what the devices are doing and use their activity to indirectly determine

the bus loading.