vmstat

Understanding vmstat Output

vmstat’s output is a table, presenting key metrics across many categories. The exact columns and their meanings depend on the options used, but common fields include:

• procs: Running and blocked processes.
• memory: Physical memory usage (used, free, buffered, cached).
• swap: Swap space usage (used, free).
• io: Block I/O statistics (read/write blocks, block I/O operations).
• system: System activity (interrupts, context switches).
• CPU: CPU utilization (user, system, idle, etc.).

Basic Usage: Getting a Single Snapshot

The simplest way to use vmstat is to execute it without any arguments. This provides a single snapshot of your system’s current state.

vmstat

This will give you one line of data representing the current state. To get a more detailed picture, you’ll need to use the -n option to display the number of processes running concurrently.

vmstat -n

Monitoring System Performance Over Time

To track performance changes over time, specify a delay (in seconds) and the number of samples using two numeric arguments. For example, the following command displays system statistics every 2 seconds for 5 samples:

vmstat 2 5

This provides a time series of your system’s activity, enabling you to identify trends and potential bottlenecks. Consider adding the -n for more detailed data:

vmstat -n 2 5

Focusing on Specific Metrics

For more granular analysis, use specific options. Here are a few examples:

• Detailed CPU Statistics: The -S option displays detailed CPU statistics, including the time spent in different CPU states (idle, user, system, etc.).

vmstat -S 2 5

• Expanded Memory Statistics: The -m option provides more detailed memory statistics than the standard output. Combine with other options for even richer information:

vmstat -m -S 2 5

Interpreting the Output: A Practical Example

Let’s imagine you run vmstat 2 5. You might observe something like this (actual numbers will vary depending on your system):

procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
0  0      0  10000  2000  50000    0    0    10    20  100  200  2  5 90  3  0
0  0      0  9800  2000  50200    0    0    12    22  105  205  3  6 89  2  0
0  0      0  9600  2000  50400    0    0     8    18  102  202  2  4 92  2  0
0  0      0  9400  2000  50600    0    0    15    25  108  208  4  7 87  2  0
0  0      0  9200  2000  50800    0    0    11    21  101  201  3  5 90  2  0

From this, you could infer that CPU utilization is relatively low (mostly idle), memory usage is stable, and I/O operations are moderate. However, a decrease in free memory over time might indicate a potential problem requiring further investigation. Analyzing such patterns is key to effective system administration.

Remember to adjust the options and intervals according to your specific needs and system characteristics. vmstat’s flexibility makes it a powerful tool for anyone looking to gain a deeper understanding of their Linux system’s performance.