iostat

Understanding iostat Output

The basic syntax of iostat is simple:

iostat

Running this command without any arguments will display aggregate I/O statistics for all devices. The output might look like this (your numbers will differ):

Linux 5.15.0-76-generic (my-server)    10/26/2023  _x86_64_        (2 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.25    0.00    0.23    0.01    0.00   99.51

Device:         rrqm/s   wrqm/s     r/s     w/s    rMB/s    wMB/s avgrq-sz avgqu-sz     await  svctm  %util
sda              0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00     0.00   0.00   0.00

Let’s break down some key columns:

• Device: The name of the block device (e.g., sda, sdb, /dev/nvme0n1).
• r/s: Reads per second.
• w/s: Writes per second.
• rMB/s: Megabytes read per second.
• wMB/s: Megabytes written per second.
• avgqu-sz: Average queue length. A high value suggests I/O bottlenecks.
• await: Average time (in milliseconds) spent waiting for I/O operations.
• %util: Percentage of time the device is busy servicing I/O requests. A value close to 100% indicates saturation.

Monitoring I/O Over Time

To monitor I/O performance over a period of time, use the -x (extended statistics) and -t (timestamp) options, along with the -I option to include detailed information on the CPU usage:

iostat -x -t -I -d 1 5

This command will display extended statistics every second for 5 seconds. The -d flag specifies that only disk devices should be displayed. This provides a more granular view of I/O activity, allowing you to identify short-lived spikes or sustained periods of high load.

Focusing on Specific Devices

If you’re only interested in the performance of a particular device, specify it as an argument:

iostat -x /dev/sda

This will only show detailed statistics for the /dev/sda device.

Advanced Usage: Analyzing Different I/O Schedulers

Different I/O schedulers (e.g., cfq, noop, deadline) impact performance differently. You can use iostat in conjunction with other commands to investigate this. However, this involves deeper system administration concepts and requires more context specific investigation.

Interpreting iostat Results for Troubleshooting

High values for avgqu-sz, await, and %util suggest potential I/O bottlenecks. For example, if %util is consistently near 100% for a specific device, it indicates that the disk is saturated and unable to keep up with the I/O requests. This might necessitate upgrading the disk, optimizing database queries, or investigating other performance issues within your application. Analyzing the r/s and w/s values, in conjunction with the average queue size and wait times, can help to pinpoint whether read or write operations are the limiting factor.

By carefully observing the output of iostat and understanding its metrics, you can gain a better understanding of your system’s I/O performance and effectively address any bottlenecks impacting application performance and system responsiveness.