HDFS: I have under/over replicated blocks. How can I find more information?

In the case where HDFS reports there are under or over replicated blocks you can use the metasave option

$ sudo -u hdfs hdfs dfsadmin -metasave metasave-report.txt 

$ cd /var/log/hadoop-hdfs

$ cat metasave-report.txt

Metasave: Blocks waiting for replication: 125

/mapred/tmp/hdfs/.staging/job_1395354337974_1132/job.jar: blk_1073994598_253980 (replicas: l: 6 d: 0 c: 0 e: 0)  11.10.1.9:50010 :  11.12.14.30:50010 :  1.10.14.8:50010 :  1.12.10.2:50010 :  1.20.104.94:50010 :  1.10.14.2:50010 : 

Each block will have a line such as above which contains information on its replication (see highlighted):
l: live replicas
d: decomissioned replicas
c: corrupt replicas
e: excess replicas

This information can be used to determine the next action (such as remove the block if it is corrupt).

Above we can see the block has 6 live replicas and the IP of each datanode that stores one.