When troubleshooting it is often useful to know the current disk and mount I/O. This can be seen using the following command:

This article will show how to produce a constantly updating disk usage monitor, similar to top.

Logapp root:/loglogic/data 0$ watch -n1 iostat

Every 1.0s: iostat

Linux 4.1.12-124.18.5.el6uek.x86_64 (logapp)    09/20/19        _x86_64_        (1 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
          11.06    0.28    4.61    0.10    0.00   83.94

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
sda              15.12         0.75       390.34    1413440  738435208
dm-0              1.01         0.03         8.85      65122   16741064
dm-1              0.00         0.00         0.00       2400          0
dm-2             32.33         0.41       381.25     777314  721241064

This output will be updated every second in real-time.  For a more detailed view, you can use:

Logapp root:/loglogic/data 0$ watch -n1 iostat -x

Every 1.0s: iostat -x

Linux 4.1.12-124.18.5.el6uek.x86_64 (logapp)    09/20/19        _x86_64_        (1 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
          11.06    0.28    4.61    0.10    0.00   83.94

Device:         rrqm/s   wrqm/s     r/s     w/s   rsec/s   wsec/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00    18.24    0.02   15.10     0.75   390.34    25.86     0.00    0.33    0.31    0.33   0.21   0.31
dm-0              0.00     0.00    0.00    1.01     0.03     8.85     8.80     0.00    0.24    0.34    0.24   0.09   0.01
dm-1              0.00     0.00    0.00    0.00     0.00     0.00     8.92     0.00    0.06    0.06    0.00   0.06   0.00
dm-2              0.00     0.00    0.01   32.32     0.41   381.25    11.80     0.01    0.42    0.30    0.42   0.09   0.31

The specific fields to focus on will differ depending on the type of issue being troubleshoot but important ones to be aware of are :
- the r/s and w/s for knowing how many read and write requests per second, respectively, are being issued
- the r_await and w_await for knowing the delay between the read and write requests being issued and when they are served
- %util for knowing the percentage of elapsed time during which I/O requests were issued to the device.

The command man iostat can provide much more information for how to use the command and understand the information it provides.