HDFS-5806 changed the socket read timeout for the balancer connection to DN to 60 seconds. This works as long as balancer bandwidth is such that it's safe to assume that the DN will easily complete the operation within this time. Obviously this isn't a good assumption. When this assumption isn't valid, the balancer will timeout the cmd BUT it will then be out-of-sync with the datanode (balancer thinks the DN has room to do more work, DN is still working on the request and will fail any subsequent requests with "threads quota exceeded errors"). This causes expensive NN traffic via getBlocks() and also causes lots of WARNS int the balancer log.
Unfortunately the protocol is such that it's impossible to tell if the DN is busy working on replacing the block, OR is in bad shape and will never finish.
So, in the interest of a small change to deal with both situations, I propose the following two changes:
- Crank of the socket read timeout to 20 minutes
- Delay looking at a node for a bit if we did timeout in this way (the DN could still have xceiver threads working on the replace