online upgrade 2.0.0 -> 2.0.1 rebalance exited with {unexpected_exit, {'EXIT',<0.2421.1>, {{badmatch, [{'EXIT', {timeout, {gen_server,call, [<0.2412.1>,had_backfill,30000]}}}]}, [{ns_single_vbucket_mover, '-wait_backfill_determination/1-fun-1-',1}]}}}

steps:
1. 3 1976 nodes in cluster with 3 different buckets, 1 view in each bucket, start upload data for each bucket through mcsoda:
ip:10.3.121.112
ip:10.3.121.114
ip:10.3.121.115

2. add 2 139build nodes:10.3.121.116, 10.3.121.117 and remove 10.3.121.114,10.3.121.115 ( swap rebalance)

rebalance exit with progress ~ 40% when each bucket contains only ~400K at the time

<0.2363.1> exited with {unexpected_exit,
{'EXIT',<0.2421.1>,
{{badmatch,
[{'EXIT',
{timeout,

{gen_server,call, [<0.2412.1>,had_backfill,30000]}

}}]},
[

{ns_single_vbucket_mover, '-wait_backfill_determination/1-fun-1-',1}

]}}}

also please note, that error message doesn't contain any mention of rebalance ( <0.2363.1> exited with {unexpected_exit)

I also noticed that during the rebalance stats graphics do not show up on, and statistics for some buckets did not change at all. for example, the number of operations per second for default bucket was always 526 operations per second.
only when rebalance failed and I stopped mcsoda loading all stats were updated