As disks are getting larger and more plentiful, we're seeing DNs with multiple millions of blocks on a single machine. When page cache space is tight, block reports can take multiple minutes to generate. Currently, during the scanning of the data directories to generate a report, the FSVolumeSet lock is held. This causes writes and reads to block, timeout, etc, causing big problems especially for clients like HBase.
This JIRA is to explore some of the ideas originally discussed in
HADOOP-4584 for the 0.20.20x series.