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vmstat

The vmstat command provides information about system processes, memory, paging, block device I/O, traps, disks, and CPU activity

Basic Usage

Summary of System Resource Usage

Run the command with no arguments to get a summary of the current system.

vmstat

To see updates in real-time: 

vmstat 1 5

  • 1: Update every second
  • 5: Run 5 times every x number of seconds (1 in this case)

Getting Disk Statistics

You can see a summary of disk statistics by using the -D flag: 

vmstat -D
Or, get a more detailed view of disk statistics with -d: 
vmstat -d

vmstat Output

Understand the output of vmstat (see below for more detailed explanations).

  • Procs (Processes):

    • r: The number of processes waiting for run time.
    • b: The number of processes blocked waiting for I/O to complete.

  • Memory:

    • swpd: The amount of virtual memory used.
    • free: The amount of idle memory.
    • buff: The amount of memory used as buffers.
    • cache: The amount of memory used as cache.

  • Swap:

    • si: Amount of memory swapped in from disk (/s).
    • so: Amount of memory swapped out to disk (/s).

  • IO:

    • bi: Blocks received from a block device (blocks/s).
    • bo: Blocks sent to a block device (blocks/s).

  • System:

    • in: The number of interrupts per second, including the clock.
    • cs: The number of context switches per second.

  • CPU (shown as percentages of total CPU time):

    • us: Time spent running non-kernel code. (user time, including nice time)
    • sy: Time spent running kernel code. (system time)
    • id: Time spent idle.
    • wa: Time spent waiting for IO.
    • st: Time stolen from a virtual machine. Relevant in virtualized environments.

Procs (Processes)

  • r: This is the number of programs (processes) waiting for their turn to run on the computer's processor (CPU).
    • This is like how many people are waiting in line at a shop.
  • b: This is the number of processes that are sleeping and cannot be woken up until they get a specific piece of data or event.
    • Like a chef waiting for an ingredient that's not yet delivered.

Memory

  • swpd: This represents the amount of virtual memory used.
    • Virtual memory is like an extension of your computer's real memory (RAM), but it's slower because it uses part of your hard drive.
  • free: This shows how much memory is not being used right now.
  • buff: Buffers are temporary holding spots for data that's being transferred between different programs or between programs and hardware.
  • cache: Cache is a small storage area that keeps copies of data from frequently used parts of your memory to speed up access.
    • Like keeping your most-used books on your desk for quick access instead of fetching them from the bookshelf each time.

Swap

Swap is used when there's not enough memory (RAM) for all the processes that are currently running. Swap is way slower than RAM. Even with a fast SSD, it's still orders of magnitude slower than your system's memory.

  • si and so: These indicate how much data is being moved between your RAM and your hard drive's swap space (a designated area used as virtual memory).
    • si: swap-in (loading into RAM)
    • so: swap-out (moving out of RAM to make space).
    • Like moving books between your desk and a storage box under the desk to make room for what you currently need.

IO (Input/Output)

  • bi and bo: Block device input and block device output.
    • These show how much data is being read (bi) from or written (bo) to block devices like hard drives.
    • A block device is a piece of hardware that stores data in fixed-size chunks (blocks).
    • bi is loading stuff in, and bo is taking stuff out.

System

  • in: Interrupts. This is the number of interrupts per second.
    • An interrupt is a signal to the CPU that it needs to stop and do something else.
  • cs: Context Switch. This is the number of context switches per second.
    • A context switch occurs when the CPU switches from running one process to another.
    • i.e., it's working on a task, then stopping to work on a different task, and later returning to the original task.

CPU

  • us: User. Time the CPU spends running programs' code.
  • sy: System. Time the CPU spends running system (kernel) code, like managing memory or devices.
  • id: Idle. Time the CPU is doing nothing.
  • wa: Wait. Time the CPU waits for IO operations to complete before doing more work.
  • st: Stolen. Time "stolen" from this virtual machine by the hypervisor for other tasks (in virtual environments).