FAQ
Datanode can have more than one copy of same block when a failed disk is coming back in datanode
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Key: HDFS-1940
URL: https://issues.apache.org/jira/browse/HDFS-1940
Project: Hadoop HDFS
Issue Type: Bug
Components: data-node
Affects Versions: 0.20.204.0
Reporter: Rajit


There is a situation where one datanode can have more than one copy of same block due to a disk fails and comes back after sometime in a datanode. And these duplicate blocks are not getting deleted even after datanode and namenode restart.

This situation can only happen in a corner case , when due to disk failure, the data block is replicated to other disk of the same datanode.


To simulate this scenario I copied a datablock and the associated .meta file from one disk to another disk of same datanode, so the datanode is having 2 copy of same replica. Now I restarted datanode and namenode. Still the extra data block and meta file is not deleted from the datanode

[hdfs@gsbl90192 rajsaha]$ ls -l `find /grid/{0,1,2,3}/hadoop/var/hdfs/data/current -name blk_*`
-rw-r--r-- 1 hdfs users 7814 May 13 21:05 /grid/1/hadoop/var/hdfs/data/current/blk_1727421609840461376
-rw-r--r-- 1 hdfs users 71 May 13 21:05 /grid/1/hadoop/var/hdfs/data/current/blk_1727421609840461376_579992.meta
-rw-r--r-- 1 hdfs users 7814 May 13 21:14 /grid/3/hadoop/var/hdfs/data/current/blk_1727421609840461376
-rw-r--r-- 1 hdfs users 71 May 13 21:14 /grid/3/hadoop/var/hdfs/data/current/blk_1727421609840461376_579992.meta

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