5. I/O Monitoring

Effective I/O monitoring is crucial for maintaining a healthy and responsive Linux system. A powerful command-line tool for this task is iostat, which provides detailed reports on both CPU and disk activity.

Running the iostat command generates a snapshot of your system's performance metrics.

pete@icebox:~$ iostat
Linux 3.13.0-39-lowlatency (icebox)     01/28/2016      _i686_  (1 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           0.13    0.03    0.50    0.01    0.00   99.33

Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
sda               0.17         3.49         1.92     385106     212417

The output is divided into two main sections. Let's break them down.

Understanding CPU Metrics

The first report details CPU utilization, providing insight into how the processor is spending its time.

• %user: Percentage of CPU time spent executing user-level (application) processes.
• %nice: Percentage of CPU time spent on user-level processes with a modified (nice) priority.
• %system: Percentage of CPU time spent executing system-level (kernel) processes.
• %iowait: Percentage of time the CPU was idle while waiting for an outstanding disk I/O request to complete. High values here can indicate a storage bottleneck.
• %steal: In a virtualized environment, this is the percentage of time a virtual CPU waits for a real CPU while the hypervisor is servicing another virtual processor.
• %idle: Percentage of time the CPU was idle and not waiting for any disk I/O requests.

Analyzing Disk Utilization

The second report focuses on device-level I/O monitoring, showing how data is being transferred to and from your storage devices.

• tps: Transfers per second issued to the device. A transfer is an I/O request, and multiple logical requests can be combined into a single one.
• kB_read/s: The amount of data read from the device, expressed in kilobytes per second.
• kB_wrtn/s: The amount of data written to the device, expressed in kilobytes per second.
• kB_read: The total number of kilobytes read from the device since the last reboot.
• kB_wrtn: The total number of kilobytes written to the device since the last reboot.