A system is
or has an I/O
if the peripheral devices (hard disk, tape, and so
on) cannot transfer data as fast as
the system requests it.
This causes processes to be put to sleep,
``waiting for I/O'', and leaves
the CPU(s) idle for much of the time.
if the system is disk I/O bound run sar -u
(or cpusar -u for SMP)
and look at the
This displays the percentage of time that each CPU
spends waiting for I/O to complete while there are
If this value is high
then it is possible that I/O is not keeping up with
the rest of the system.
(You should not always assume that
there is a problem with disks; for example,
%wio might be high because a tape drive is
Other indications of a disk I/O
bottleneck can be seen using sar -d
(or mpsar -d for SMP).
Note that sar -d can be also be used to view the
activity of block I/O devices including
hard disk drives, SCSI tape drives, and floppy disks.
If the values for
avque are both
consistently high then the devices cannot keep up with the requests to
Devices such as floppy disks and some older types of tape drive are
As these devices are generally infrequently used -- for
system backup, software installation, and so on -- there is
little that performance tuning can usefully accomplish.
The value of
displayed by sar -d
can be combined with
%busy to give an
indication of the maximum I/O
throughput of a disk, and
may suggest where a I/O bottleneck can occur:
Maximum disk throughput (KB/s) =
blks/s 50 /
High values of the ratio of
also suggest that the device is saturated with requests.
If the number of transfers,
is high but the amount of data being transferred,
blks/s, is low, it may be possible to
modify the application to transfer larger amounts of data
less frequently. This should reduce the number of requests
for the disk and reduce contention for it.
The read and write hit rates (
%wcache) shown by sar -b should
show high values.
If these values fall, the system is having to access blocks
on disk (or other block devices) rather than in the
If this happens, increasing the size of the buffer cache
may help to alleviate a disk I/O bottleneck.
A low hit rate for the namei cache could lead to the disk
being accessed more often in order to convert pathname components
to inode numbers.
If sar -n displays results showing that
hit % is consistently low
then the namei cache for the corresponding filesystem type
is too small. It is not possible to give
a general definition of what is a low value since this
depends on the application mix that you run on your system.
Because the performance of the namei cache does not depend linearly
on its size, you will find that improving cache hit
rates that are already high requires a significantly greater
cache size. For example, you might have to
increase the size of the namei cache by 30% in order to
increase the namei cache hit rate from 90% to 95% giving
a relative gain of 5.6%.
Note that namei caching is only performed for pathname components that are 14 characters or less in length. To take advantage of the namei cache, you should use directory names and filenames that are less than 15 characters long.
The following table is a summary of the commands that can be used to determine if a system is I/O bound:
Identifying an I/O-bound system
|[cpu]sar -u||%wio||percent of time that the CPU spends waiting for I/O with no processes to run|
|[mp]sar -b||bread/s||average number of blocks read into the buffer cache per second|
|bwrit/s||average number of blocks written from the buffer cache per second|
|%rcache||percent of read disk block requests satisfied by reading the buffer cache|
|%wcache||percent of write disk block requests satisfied by writing to the buffer cache|
|[mp]sar -d||avque||average number of requests on queue waiting for device per second|
|avserv||average time transfer takes to complete in milliseconds|
|blks/s||number of 512 byte blocks transferred to and from the device per second|
|%busy||percent of time device was servicing transfer request|
|r+w/s||number of read/write transfers to device per second|
|[mp]sar -n||hit %||namei cache hit rate as a percentage|