The low-water mark for AUFS/UFS/diskd cache object eviction by
the cache_replacement_policy algorithm.
Removal begins when the swap (disk) usage of a cache_dir is
above this low-water mark and attempts to maintain utilization
near the low-water mark.
As swap utilization increases towards the high-water mark set
by cache_swap_high object eviction becomes more aggressive.
The value difference in percentages between low- and high-water
marks represent an eviction rate of 300 objects per second and
the rate continues to scale in aggressiveness by multiples of
this above the high-water mark.
Defaults are 90% and 95%. If you have a large cache, 5% could be
hundreds of MB. If this is the case you may wish to set these
numbers closer together.
See also cache_swap_high and cache_replacement_policy