Not sure what you mean. Do you want to know WHY blocks get under replicated? There are different possibilities for a block to vanish but by and large its simple: a) The block replica doesn't get written in the first place This happens during network or node failure during a write. HDFS will still return the write of a block as successful as long as one of the block replica writes was successful . So if for example the third designated datanode dies during the write process the write is still successful but the block will be under replicated. The write process doesn't care and they depend on the Namenode to schedule a copy later on. b) The block replicas get deleted later. That can have lots of different reasons. Node dies, drive dies, you delete a block file in the drive. Blocks after all are simple bog standard Linux files with a name blkxxxx which is the block id. They can also get corrupted ( HDFS does CRC checks regularly and blocks that are corrupted will be replaced with a healthy copy. And many more ... So perhaps you should be a bit more specific with your question?
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