squito commented on issue #23510: [SPARK-26590][CORE] make fetch-block-to-disk 
backward compatible
URL: https://github.com/apache/spark/pull/23510#issuecomment-454233253
 
 
   Thanks for doing this @cloud-fan -- I have one high level question before 
looking more carefully at the code.  When you say:
   
   > new client <-> old server: The ChunkFetchRequest message contains the 
fetchAsStream flag, but the old server doesn't know about it and stops reading 
the message right before the fetchAsStream part. Then the old server returns 
normal chunk fetch response, and new client accept it.
   
   so the client accepts it, but will the client stream to disk still, or will 
it fallback to still fetching to memory?  It seems it should be possible to 
stream to disk, as the server is really sending virtually the same bytes either 
way (just a different header, more or less).  That said, I think the actually 
plumbing is rather complicated to update (this is something @attilapiros looked 
at earlier).
   
   I can see about testing this with old versions of the shuffle server -- and 
especially if you think this makes stream-to-disk work even with the old 
shuffle server, than we can also test out shuffling large blocks too.

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