# vmstat — Monitor Memory, Swap and CPU in Real Time > Practical guide to vmstat — watch memory, swap, I/O wait and CPU load second by second and spot system bottlenecks early. Source: https://www.jpkc.com/db/en/cheatsheets/shell-system/vmstat/ vmstat (virtual memory statistics) gives you the pulse of a Linux system in a single, compact line: process queues, memory, swap, block I/O and CPU usage at a glance. Part of the procps-ng package, it is a read-only command – perfect for quickly spotting memory pressure, swapping or I/O bottlenecks. One catch: when you run it with an interval (for example `vmstat 2`), the first line reports averages since boot – only the lines that follow reflect current activity. ## Basic Usage `vmstat` — Show a single snapshot of system statistics since boot. ```bash vmstat ``` `vmstat INTERVAL` — Show statistics every INTERVAL seconds, continuously. ```bash vmstat 1 ``` `vmstat INTERVAL COUNT` — Show statistics every INTERVAL seconds, COUNT times. ```bash vmstat 1 10 ``` `vmstat -w` — Wide output — wider columns for large values. ```bash vmstat -w 1 ``` `vmstat -t` — Append timestamp to each line. ```bash vmstat -t 1 ``` ## Memory Statistics `vmstat -s` — Show detailed memory statistics in a table format. ```bash vmstat -s ``` `vmstat -S M` — Show values in megabytes instead of kilobytes. ```bash vmstat -S M 1 ``` `vmstat -S k` — Show values in kilobytes (default). ```bash vmstat -S k 1 ``` `vmstat -a` — Show active and inactive memory instead of buffer and cache. ```bash vmstat -a 1 ``` ## Disk Statistics `vmstat -d` — Show disk statistics: reads, writes, I/O time per device. ```bash vmstat -d ``` `vmstat -D` — Show disk summary statistics. ```bash vmstat -D ``` `vmstat -p PARTITION` — Show statistics for a specific partition. ```bash vmstat -p sda1 ``` ## Other Options `vmstat -f` — Show the number of forks (process creations) since boot. ```bash vmstat -f ``` `vmstat -n` — Display the header only once (not periodically). Useful for logging. ```bash vmstat -n 1 60 > vmstat.log ``` ## Output Columns Explained `r (procs)` — Number of runnable processes (waiting for CPU). High values = CPU contention. `b (procs)` — Number of processes in uninterruptible sleep (usually waiting for I/O). `swpd (memory)` — Amount of swap memory used (KB). Non-zero = memory pressure. `free (memory)` — Amount of idle/free memory (KB). `buff (memory)` — Amount of memory used as kernel buffers (KB). `cache (memory)` — Amount of memory used as page cache (KB). `si (swap)` — Memory swapped in from disk per second (KB/s). Non-zero = active swapping. `so (swap)` — Memory swapped out to disk per second (KB/s). Non-zero = memory pressure. `bi (io)` — Blocks received from block devices per second. `bo (io)` — Blocks sent to block devices per second. `in (system)` — Interrupts per second. `cs (system)` — Context switches per second. `us (cpu)` — Time spent running user code (%). `sy (cpu)` — Time spent running kernel/system code (%). `id (cpu)` — Idle time (%). Low = CPU busy. `wa (cpu)` — Time waiting for I/O (%). High = I/O bottleneck. `st (cpu)` — Time stolen by hypervisor (%). Non-zero = VM resource contention. ## Common Patterns `vmstat -w 1` — Real-time system overview with wide columns. The go-to command. ```bash vmstat -w 1 ``` `vmstat -S M -w 1` — Real-time monitoring with values in megabytes. ```bash vmstat -S M -w 1 ``` `vmstat 1 | awk '$7 > 0 || $8 > 0'` — Watch for swap activity (si/so columns non-zero). ```bash vmstat 1 | awk '$7 > 0 || $8 > 0' ``` `vmstat -n -t 1 3600 > vmstat.log` — Log system stats for one hour (1 sample/sec). ```bash vmstat -n -t 1 3600 > vmstat.log ``` `vmstat 1 | awk 'NR>2 && $1>4 {print "CPU overloaded: r="$1}'` — Alert when runnable processes exceed CPU count. ```bash vmstat 1 | awk 'NR>2 && $1>4 {print "CPU overloaded: r="$1}' ``` ## Conclusion vmstat is one of the fastest diagnostic tools around: one command, one line, and you know whether your system is under memory, CPU or I/O pressure. Remember the key columns: sustained non-zero `si`/`so` means real memory pressure from swapping, a persistently high `r` queue points to a CPU shortage and high `wa` signals an I/O bottleneck. Always pass an interval such as `vmstat 2` and ignore the first line, since it only reports the average since boot. For per-device or per-process detail, pair vmstat with `iostat`, `free` and `top`. ## Further Reading - [manpages.debian.org: vmstat(8)](https://manpages.debian.org/bookworm/procps/vmstat.8.en.html) – the official manual page from the procps package - [Brendan Gregg: Linux Performance](https://www.brendangregg.com/linuxperf.html) – widely referenced overview of Linux performance-analysis tools including vmstat ## Related Commands - [iostat](https://www.jpkc.com/db/en/cheatsheets/shell-system/iostat/) – detailed per-device disk and I/O utilisation - [free](https://www.jpkc.com/db/en/cheatsheets/shell-system/free/) – quickly show free and used memory - [top](https://www.jpkc.com/db/en/cheatsheets/shell-system/top/) – watch processes and resource usage in real time