Even though iostat can show cumulative statistics on CPU utilisation …

root:/tmp> iostat ­-c 5 2
Linux 4.1.8­100.fc21.x86_64 (envy17)      10/23/2015     _x86_64_   (8 CPU)
.
avg­cpu: %user  %nice   %system   %iowait   %steal   %idle
.        7.42   1.00      2.43      1.52     0.00   87.63
.
avg­cpu: %user  %nice   %system   %iowait   %steal   %idle
.        2.93   0.00      0.53      1.75     0.00   94.79

… its main use is to extract I/O stats …

root:/tmp> iostat 5 2
Linux 4.1.8­100.fc21.x86_64 (envy17)    10/23/2015    _x86_64_   (8 CPU)
.
avg­cpu: %user   %nice   %system   %iowait   %steal   %idle
.        7.41    1.00      2.43      1.52     0.00   87.65
.
Device:      tps      kB_read/s      kB_wrtn/s      kB_read      kB_wrtn
sda         5.95           7.08          93.52      4747376     62743664
sdb         5.64           2.85         100.03      1914758     67112760
md127       5.75           6.80          91.48      4564249     61373872
dm­0         0.08           1.61           0.41      1076852       275872
dm­1         0.13           1.84           1.49      1236701      1002040
dm­2         0.00           0.07           0.01        48153         6368
dm­3         0.00           0.02           0.01        11721         4644
.
avg­cpu: %user   %nice   %system   %iowait   %steal   %idle
.        3.28    0.00      1.18      3.71     0.00   91.83
.
Device:      tps      kB_read/s      kB_wrtn/s      kB_read      kB_wrtn
sda       152.60           0.00        2648.60            0        13243
sdb       156.00           0.00        3450.20            0        17251
md127     156.20           0.00        2648.00            0        13240
dm­0         0.00           0.00           0.00            0            0
dm­1         0.00           0.00           0.00            0            0
dm­2         0.00           0.00           0.00            0            0
dm­3         0.00           0.00           0.00            0            0

When iostat is executed without any options, the default flags -c and -d are used. So “iostat ­-d 5 2” would show the same output as the example above minus the average CPU part.

Usually iostat is used to monitor I/O workload on disk devices and is run a number of times a few seconds apart. But we can also run it without specifying any iterations/wait_intervals:

root:/tmp> iostat
Linux 4.1.8­100.fc21.x86_64 (envy17)    10/23/2015    _x86_64_   (8 CPU)
.
avg­cpu:  %user   %nice   %system   %iowait   %steal   %idle
.         7.41    1.00      2.43      1.52     0.00   87.65
.
Device:      tps      kB_read/s      kB_wrtn/s      kB_read      kB_wrtn
sda         6.02           7.09          94.79      4763116     63636715
sdb         5.72           2.86         101.35      1919430     68046611
md127       5.82           6.81          92.74      4575261     62261928
dm­0         0.08           1.60           0.41      1076852       275912
dm­1         0.13           1.86           1.51      1246101      1011140
dm­2         0.00           0.07           0.01        48153         6368
dm­3         0.00           0.02           0.01        11721         4644

The example above shows the average tps (transfers or I/Os per second), Kbytes_read/sec and
Kbytes_written/sec, total Kbytes read and total Kbytes written since boot. So most of the time they are a bit less than useless stats.

We will normally run iostat specifying a waiting interval and a number of iterations:

root:/tmp> iostat ­-d 5          → will extract the stats every 5 seconds forever
root:/tmp> iostat ­-d 5 1000     → will extract the stats 1000 times and exit

The most useful iostat options are:

• -m      → shows Mbytes rather than the default Kbytes

  -t       → shows the current timestamp at every iteration

  -x      → shows extended statistics (queue size & length, avg wait time, etc)-

  -N      → shows mapped device names

  -y      → omits the 1 st iteration showing stats since boot

  -z      → omits those devices without any I/O activity

  -p <dev|ALL> → show stats for device and its partitions (default is just device)

  -j ID|LABEL|UUID → show the device by ID/LABEL/UUID rather than its device name

  -g group <dev1,dev2,… ALL> → gathers cumulative stats for all the devices in group

  -T     → shows stats of group defined with -­g flag and omits individual devices

Let’s see some examples and how the output changes with the different flags. Without any flags we get both CPU and disk I/O stats:

root:/tmp> iostat 2
Linux 4.1.8­100.fc21.x86_64 (envy17)    10/23/2015     _x86_64_    (8 CPU)
.
avg­cpu: %user   %nice   %system   %iowait   %steal   %idle
.        3.38    0.00      0.38      3.13     0.00   93.12
.
Device:      tps      kB_read/s      kB_wrtn/s      kB_read      kB_wrtn
sda       145.00           0.00        2198.25            0         4396
sdb       141.00           0.00        2182.25            0         4364
md127     141.50           0.00        2182.00            0         4364
dm­0         0.00           0.00           0.00            0            0
dm­1         0.00           0.00           0.00            0            0
dm­2         0.00           0.00           0.00            0            0
dm­3         0.00           0.00           0.00            0            0

With the -c flag we get CPU stats:

root:/tmp> iostat -c 2
Linux 4.1.8­100.fc21.x86_64 (envy17)    10/23/2015     _x86_64_    (8 CPU)
.
avg­cpu: %user   %nice   %system   %iowait   %steal   %idle
.        3.38    0.00      0.38      3.13     0.00   93.12

With the -d flag we get disk I/O stats:

root:/tmp> iostat -­d 2
Linux 4.1.8­100.fc21.x86_64 (envy17)    10/23/2015    _x86_64_ (8 CPU)
.
Device:      tps      kB_read/s      kB_wrtn/s      kB_read      kB_wrtn
sda       109.00           0.00         2038.75           0         4077
sdb       108.50           0.00         2036.75           0         4073
md127     108.50           0.00         2038.00           0         4076
dm­0         0.00           0.00            0.00           0            0

With -y the first set of “since-boot” figures is ommitted. The -m flag shows data in mbytes:

root:/tmp> iostat ­-dym 2
Linux 4.1.8­100.fc21.x86_64 (envy17)    10/23/2015    _x86_64_   (8 CPU)
.
Device:      tps      MB_read/s      MB_wrtn/s      MB_read      MB_wrtn
sda       143.00           0.00           2.17            0            4
sdb       143.00           0.00           2.17            0            4
md127     142.00           0.00           2.17            0            4
dm­0         0.00           0.00           0.00            0            0

With -t (timestamp) and -x (extended stats) we get …

root:/tmp> iostat ­-dymxt 2
Linux 4.1.8­100.fc21.x86_64 (envy17)  10/23/2015   _x86_64_   (8 CPU)
.
10/23/2015 11:06:58 AM
Device: rrqm/s wrqm/s   r/s   w/s  rMB/s  wMB/s avgrq­sz avgqu­sz await r_await w_await svctm %util
sda       0.00   0.00  0.00 97.50   0.00   1.94   40.78    0.67  6.85    0.00    6.85  2.06 20.10
sdb       0.00   0.50  0.00 98.00   0.00   1.95   40.69    3.85 39.24    0.00   39.24  7.56 74.05
md127     0.00   0.00  0.00 93.50   0.00   1.94   42.48    0.00  0.00    0.00    0.00  0.00  0.00
dm­0       0.00   0.00  0.00  0.00   0.00   0.00    0.00    0.00  0.00    0.00    0.00  0.00  0.00
dm­1       0.00   0.00  0.00  0.00   0.00   0.00    0.00    0.00  0.00    0.00    0.00  0.00  0.00
dm­2       0.00   0.00  0.00  0.00   0.00   0.00    0.00    0.00  0.00    0.00    0.00  0.00  0.00
dm­3       0.00   0.00  0.00  0.00   0.00   0.00    0.00    0.00  0.00    0.00    0.00  0.00  0.00

With -N (mapped device name) we get …

root:/tmp> iostat ­-dymxtN 2
Linux 4.1.8­100.fc21.x86_64 (envy17)  10/23/2015   _x86_64_   (8 CPU)
.
10/23/2015 11:06:58 AM
Device:     rrqm/s wrqm/s   r/s   w/s  rMB/s  wMB/s avgrq­sz avgqu­sz await r_await w_await svctm %util
sda           0.00   0.00  0.00 97.50   0.00   1.94   40.78    0.67  6.85    0.00    6.85  2.06 20.10
sdb           0.00   0.50  0.00 98.00   0.00   1.95   40.69    3.85 39.24    0.00   39.24  7.56 74.05
md127         0.00   0.00  0.00 93.50   0.00   1.94   42.48    0.00  0.00    0.00    0.00  0.00  0.00
vg-root       0.00   0.00  0.00  0.00   0.00   0.00    0.00    0.00  0.00    0.00    0.00  0.00  0.00
vgdata-docs   0.00   0.00  0.00  0.00   0.00   0.00    0.00    0.00  0.00    0.00    0.00  0.00  0.00
vgdata-music  0.00   0.00  0.00  0.00   0.00   0.00    0.00    0.00  0.00    0.00    0.00  0.00  0.00
vgdata-other  0.00   0.00  0.00  0.00   0.00   0.00    0.00    0.00  0.00    0.00    0.00  0.00  0.00

If we want the same as above but specifying just to devices to show stats for …

root:/tmp> iostat ­-dymxtN sda sdb 2
Linux 4.1.8­100.fc21.x86_64 (envy17)  10/23/2015   _x86_64_   (8 CPU)
.
10/23/2015 11:06:58 AM
Device:     rrqm/s wrqm/s   r/s   w/s  rMB/s  wMB/s avgrq­sz avgqu­sz await r_await w_await svctm %util
sda           0.00   0.00  0.00 97.50   0.00   1.94   40.78    0.67  6.85    0.00    6.85  2.06 20.10
sdb           0.00   0.50  0.00 98.00   0.00   1.95   40.69    3.85 39.24    0.00   39.24  7.56 74.05

If we want the same as above but showing stats for the 2 devices and its partitions, then we use -p:

root:/tmp> iostat ­-dymxtN ­-p sda ­-p sdb 2
Linux 4.1.8­100.fc21.x86_64 (envy17)   10/23/2015   _x86_64_  (8 CPU)
.
10/23/2015 11:17:52 AM
Device: rrqm/s wrqm/s   r/s   w/s  rMB/s  wMB/s  avgrq­sz  avgqu­sz  await  r_await  w_await  svctm  %util
sda      0.00    0.00  0.00  1.00   0.00   0.00     1.00     0.01  14.50     0.00    14.50  14.50   1.45
sda1     0.00    0.00  0.00  0.00   0.00   0.00     0.00     0.00   0.00     0.00     0.00   0.00   0.00
sda2     0.00    0.00  0.00  0.00   0.00   0.00     0.00     0.00   0.00     0.00     0.00   0.00   0.00
sda3     0.00    0.00  0.00  1.00   0.00   0.00     1.00     0.01  14.50     0.00    14.50  14.50   1.45
sda4     0.00    0.00  0.00  0.00   0.00   0.00     0.00     0.00   0.00     0.00     0.00   0.00   0.00
sdb      0.00    0.00  0.00  1.00   0.00   0.00     1.00     0.07  71.00     0.00    71.00  71.00   7.10
sdb1     0.00    0.00  0.00  0.00   0.00   0.00     0.00     0.00   0.00     0.00     0.00   0.00   0.00
sdb2     0.00    0.00  0.00  1.00   0.00   0.00     1.00     0.07  71.00     0.00    71.00  71.00   7.10
sdb3     0.00    0.00  0.00  0.00   0.00   0.00     0.00     0.00   0.00     0.00     0.00   0.00   0.00

If we want the same as above but ommitting idle devices, then we use -z:

root:/tmp> iostat ­-dymxtNz ­-p sda -­p sdb 2
Linux 4.1.8­100.fc21.x86_64 (envy17)   10/23/2015   _x86_64_   (8 CPU)
.
10/23/2015 11:21:47 AM
Device:  rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq­sz avgqu­sz await r_await w_await svctm %util
sda      0.00    0.00  0.00  1.00   0.00   0.00     1.00     0.01  14.50     0.00    14.50  14.50   1.45
sda3     0.00    0.00  0.00  1.00   0.00   0.00     1.00     0.01  14.50     0.00    14.50  14.50   1.45
sdb      0.00    0.00  0.00  1.00   0.00   0.00     1.00     0.07  71.00     0.00    71.00  71.00   7.10
sdb2     0.00    0.00  0.00  1.00   0.00   0.00     1.00     0.07  71.00     0.00    71.00  71.00   7.10

We can also show the devices by their ID with -j id:

root:/tmp> iostat ­-dym ­-j id 2
Linux 4.1.8­100.fc21.x86_64 (envy17)   10/23/2015   _x86_64_  (8 CPU)
.
Device:       tps      MB_read/s      MB_wrtn/s      MB_read      MB_wrtn
ata­-HGST_HTS541010A9E680_J84000CHG28WSP
134.50          0.00           2.14            0            4
ata­-TOSHIBA_MQ01ABD100_738RT2NYT
135.00          0.00           2.14            0            4
lvm­-pv-­uuid-­06whsq-­Y6uc­-5rtl­-0NrM-­NN7D-­HosS-­JleYFN
135.00          0.00           2.14            0            4
dm­-name­-vg-­root
0.00          0.00           0.00            0            0
dm­-name­-vg-data­-docs
0.00          0.00           0.00            0            0
dm­-name­-vg-data­-music
0.00          0.00           0.00            0            0
dm­-name-­vg-data-­other
0.00          0.00           0.00            0            0

Let’s show now the devices by their label with -j label:

root:/tmp> iostat ­-dym -­j label 2
Linux 4.1.8­100.fc21.x86_64 (envy17)   10/23/2015   _x86_64_  (8 CPU)
.
Device:       tps      MB_read/s      MB_wrtn/s      MB_read      MB_wrtn
sda
.          136.50           0.00           2.14            0            4
sdb
.          143.50           0.00           2.17            0            4
md127
.          136.50           0.00           2.14            0            4
LVROOT
.            0.00           0.00           0.00            0            0
DOCS
.            0.00           0.00           0.00            0            0

And now by their UUID with -j uuid:

root:/tmp> iostat ­-dym -­j uuid 2
Linux 4.1.8­100.fc21.x86_64 (envy17)   10/23/2015   _x86_64_  (8 CPU)
.
Device:       tps      MB_read/s      MB_wrtn/s      MB_read      MB_wrtn
sda
.          136.50           0.00           2.14            0            4
sdb
.          143.50           0.00           2.17            0            4
md127
.          136.50           0.00           2.14            0            4
85ec848a-­c542-­4a3a-­96ac-­f2810edf310b
.            0.00           0.00           0.00            0            0
5564dc31­-bf86-­4d5a-­a3fd-­53bf5a7d13f1
.            0.00           0.00           0.00            0            0

Finally, let’s group together a few devices with -g groupname and show just those group stats:

root:/tmp> iostat -­dym ­-g dmgroup1 dm-­0 dm­-1 dm­-2 dm-­3 -­g dmgroup2 dm-­4 dm-­5 dm­-6 -­T 2
Linux 4.1.8­100.fc21.x86_64 (envy17)  10/23/2015   _x86_64_  (8 CPU)
.
Device:      tps      MB_read/s      MB_wrtn/s      MB_read      MB_wrtn
dmgroup1    0.00           0.00           0.00            0            0
dmgroup2    0.00           0.00           0.00            0            0

Most of the statistics shown by iostat are self-explanatory, but just in case let’s review what all the output fields are:

rrqm/s Number of read requests merged per second that were queued to the device.

wrqm/s Number of write requests merged per second that were queued to the device.

r/s Number (after merges) of read requests completed per second for the device.

w/s Number (after merges) of write requests completed per second for the device.

rkB/s, rMB/s Number of sectors (kilobytes, megabytes) read from the device per second.

wkB/s, wMB/s Number of sectors (kilobytes, megabytes) written to the device per second.

Avgrq-­sz Average size (in sectors) of the requests that were issued to the device.

Avgqu­-sz Average queue length of the requests that were issued to the device.

Await Average time (in milliseconds) for I/O requests issued to the device to be served. Includes time spent by the requests in queue and the time spent servicing them.

r_await Average time (in milliseconds) for read requests issued to the device to be served. Includes the time spent by the requests in queue and the time spent servicing them.

w_await Average time (in milliseconds) for write requests issued to the device to be served. Includes the time spent by the requests in queue and the time spent servicing them.

Svctm This field should not be trusted anymore and will be removed in future versions.

%util Percentage of CPU time during which I/O requests were issued to the device (bandwidth utilization for the device). Device saturation occurs when this value is close to 100% for devices serving requests serially. But for devices serving requests in parallel, such as RAID arrays and modern SSDs, this number does not reflect their performance limits.

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