Ambari on one of my test environments was warning me about under replicated blocks in the cluster:
Opening the Namenode web UI (http://test01.namenode1:50070/dfshealth.html#tab-overview) painted a different picture:
11 blocks are missing which might lead to corrupted files in the cluster.
Running file check on one of the files:
sudo -u hdfs hdfs fsck /tmp/tony/adac.json
The output reveals that the file is corrupted, was split in 4 blocks and is replicated only once on the cluster.
Nothing to do but to permanenetly delete the file:
sudo -u hdfs hdfs dfs -rm -skipTrash /tmp/tony/adac.json
Ambari wil now show 4 blocks less in the Under Replicated Blocks widget. Refreshing the Namenode web UI will also show that the corrupted blocks are removed.