keys failing to persist to disk with state "ram_but_not_disk" during rebalance

in viewquerytests.ViewQueryTests.test_employee_dataset_startkey_endkey_queries_rebalance_in

Load 200k docs
Add+Rebalance 6 nodes to cluster while running queries

Looks like what happens is the keys being moved to new servers are not always being persisted to disk.

...from test log's we report when key a did not persist and cannot be indexed by view-engine.

["query doc_id: admin0150-2008_12_27 doesn\'t exist in bucket: default", "Error expected in results for key with invalid state

{\'key_vb_state\': \'active\', \'key_last_modification_time\': \'1339537159\', \'key_data_age\': \'0\', \'key_cas\': \'7680066560307527\', \'key_exptime\': \'0\', \'key_is_dirty\': \'0\', \'key_flags\': \'0\', \'key_valid\': \'ram_but_not_disk\'}

",

There should be plenty of time to allow this as we retry the query several times and no new docs are being loaded/updated during the rebalance.

diags attached.