Official reference
highwater lowwater frequency
If BSD-style process accounting is enabled these values control its behaviour. If free space on filesystem where the log lives goes below % accounting suspends. If free space gets above % accounting resumes. determines how often do we check the amount of free space (value is in seconds). Default: 4 2 30 That is, suspend accounting if there left <= 2% free; resume it if we got >=4%; consider information about amount of free space valid for 30 seconds.
source