Github user tdas commented on the pull request:
https://github.com/apache/spark/pull/3906#issuecomment-68843161
If you are using a window operations, then previous batches data may need
to be access multiple times. If we dont put the data in WAL back in memory, the
system will have to read the data multiple times from the WAL. That's going to
be very slow, isnt it.
A smarter thing to do is to figure (based on the transformations) whether
the data is going to required multiple times or not, and accordingly store the
data in BM. Just turning it blindly setting it to false will cause a
performance regression.
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