if LowThresholdPercent > HighThresholdPercent, amountToFree at image_manager.go:208 is negative and image GC will not free memory properly.
Justification:
1) LowThresholdPercent > HighThresholdPercent implies (LowThresholdPercent * capacity / 100) > (HighThresholdPercent * capacity / 100)
2) usage is at least (HighThresholdPercent * capacity / 100)
3) amountToFree = usage - (LowThresholdPercent * capacity / 100)
Combining 1), 2) and 3) implies amountToFree can be negative.
What happens if amountToFree is negative? in freeSpace method, "for _, image := range images " loops at least once
and if everything goes fine, "delete(im.imageRecords, image.id)" is executed.
When checking for condition "if spaceFreed >= bytesToFree", it is always true as bytesToFree is negative
and spaceFreed is positive. The loop is finished, so is image GC.
At the end, only the oldest image is deleted. In situations where there is a lot of dead containers,
each container corresponing to distinct image, number of unused images can get higher.
If two new images get pulled in every 5 minutes, image GC will not work properly and will not free enough space.
Secondly, it will take a lot of time to free all unused images (hours depending on a number of unused images).
This is an incorrect configuration. Image GC should report it and refuse to work.
A lot of packages use StringSet, but they don't use anything else from
the util package. Moving StringSet into another package will shrink
their dependency trees significantly.
Integrated the imageManager into the Kubelet and applies the garbage
collection policy every 5 minutes. The default policy allows up to 90%
disk usage, after which images are garbage collected to bring limit back
down to 80%.
Fixes#157.
All images are tracked, when they were created and when they were last
used. FreeSpace() evicts these images in least recently used order,
breaking ties with oldest first.
