vmstat

The vmstat command is yet another way to show Linux performance metrics. vmstat reports a lot of information, and it can be difficult to understand what it is telling you.

The output is separated into six categories: process, memory, swap, I/O, system, and CPU. Like iostat, the first sample is an average since the last reboot. Here is a typical vmstat output:

# vmstat -m 5 3 procs                memory      swap          io     system         cpu  r  b  swpd  free buff  cache   si   so   bi  bo   in    cs us sy wa id  3  0   185    23  136   3679    0    0    9  10    0     4  8 18 10 17  1  0   185    22  136   3676    0    0  285  76 2671 28571 18 19  0 63  1  0   185    22  136   3670    0    0  333  41 2876 24252 36 17  0 46

The -m option causes the memory fields to be shown in megabytes. vmstat takes the sample interval and count parameters as many of the other performance commands do.

The process (procs) information has two columns. The r column is the number of runable processes. The b column is the number of blocked processes.

The memory section has four fields reporting how virtual memory is used. Table 3-7 lists them along with their meaning.

Table 3-7. vmstat memory Fields

Field	Description
swpd	Amount of used swap space
free	Amount of free RAM
buff	Amount of RAM used for buffers
cache	Amount of RAM used for filesystem cache

Next up are the swap metrics. Swap is an obsolete term but one that apparently won't go away. A swap involves paging all the memory consumed by a process to or from disk at one time. What a performance hit this would be. What Linux does instead is page to and from disk as needed in small chunks. Thus, we should probably stop saying memory swapped to disk and start saying memory paged to disk. Either way, Table 3-8 explains the fields.

Table 3-8. vmstat swap Fields

Field	Description
si	Amount paged from disk to memory
so	Amount paged to disk from memory

After swap are the two I/O fields. This section provides a short summary to help you determine whether Linux is busy doing a lot of disk I/O. vmstat only provides two fields showing the volume of data moving to and from disk (see Table 3-9).

Table 3-9. vmstat io Fields

Field	Description
bi	Blocks in from disk
bo	Blocks out to disk

The system fields provide a summary of how busy the Linux kernel is performing process management. Interrupts and context switches are listed (see Table 3-10). A context switch is the moving of a process off of or onto a CPU.

Table 3-10. vmstat system Fields

Field	Description
in	System interrupts
cs	Process context switches

Lastly, CPU state information is expressed as a percentage of total CPU time, as shown in Table 3-11.

Table 3-11. vmstat cpu Fields

Field	Description
us	User mode
sy	Kernel mode
wa	Waiting on I/O
id	Idle