Major Description
Currently users can specify arbitrarily big scratch_limit values. Moreover, setting scratch_limit to -1 means Impala can use unlimited scratch space.
With IMPALA-8376 it's possible to specify limits on the scrach directories. But this doesn't prevent users to try to use all the available scratch capacity.
We could add a startup flag 'max_scratch_limit' that would cap the scratch limit specified by the users.