The block mapping of block ID <=> DN location is not stored in the fsimage of HDFS, it is just kept in the running NameNode memory. Since the primary work of the block report is to report statuses of availability of each block replica on the DNs, and the checkpoint's work is to persist the namespace and namespace-associative states (such as snapshots, etc.), the checkpoint can be an operation done without problems in parallel as their two information structures will not conflict with one another. ... View more