Github user StephanEwen commented on the pull request:
https://github.com/apache/incubator-flink/pull/188#issuecomment-62372174
It uses all of them, round robin fashion. (Actually one I/O thread is
sticky with one directory, but files are distributed round robin across
threads).
The idea was to use all of the directories initially and gradually exclude
directories as they become full. This is tricky, though, as mentioned before,
because it is hard to define when a directory should be considered full.
I would guess we would need to define that relative to the space available
in other temp dirs. That way we do not introduce an absolute threshold and keep
small setups running.
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