HDFS-395 added the incremental BR feature. A concern was avoiding a race condition with ack-ing block invalidates followed by the directory scanner re-adding the block before the async delete service removes the block, possibly resulting in a full BR that includes the previously delete ack-ed block.
The solution was to batch & delay block deletion acks via a hardcoded interval of 100 * heartbeat interval (default: 5min). The delay isn't required:
- The FSDatasetSpi tracks blocks pending deletions precisely so the scanner won't re-add the block to the volume map
- Block receiving, received, and deleted go into the same pending report. A block received event will trigger an immediate IBR which includes the deletion acks. Ie. The delay is meaningless for all but a quiescent cluster
- Failing to promptly report deleted blocks on a quiescent cluster prevents the NN from updating the block maps to remove the locations