vmstat(8) swap unit

2020/01/26

When you run vmstat, you get a bunch of numbers without units:

procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 ...
 1  2 889856 150008  44788 406152   47 5145  3693  5173 2696 4030 42  3 22 33  0

You can look up the units in the man page. The si and so columns under swap are described as:

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

Additional information of what per second for most fields is above:

-S, –unit character
Switches outputs between 1000 (k), 1024 (K), 1000000 (m), or 1048576 (M) bytes. Note this does not change the swap (si/so) or block (bi/bo) fields.

Thus, it ultimately goes unspecified what the “amount” per second of swap activity is measured in.

But after this version of the man page was published, about a year and a half ago, it was discovered that this description was wrong. The -S option does change the si/so fields, and the amount is measured in that unit (with the default being K, or 1024 bytes).

A corrected version reads:

Swap
These are affected by the –unit option.
si: Amount of memory swapped in from disk (/s).
so: Amount of memory swapped to disk (/s).

and

-S, –unit character
Switches outputs between 1000 (k), 1024 (K), 1000000 (m), or 1048576 (M) bytes. Note this does not change the block (bi/bo) fields, which are always measured in blocks.

Here’s a listing of where the fixed version is distributed:

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