[jira] [Created] (HDFS-12119) Inconsistent "Number of Under-Replicated Blocks" shown on HDFS web UI and fsck report

["liuyiyang (JIRA)" <ji...@apache.org>]

liuyiyang created HDFS-12119:
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             Summary: Inconsistent "Number of Under-Replicated Blocks"  shown on HDFS web UI and fsck report
                 Key: HDFS-12119
                 URL: https://issues.apache.org/jira/browse/HDFS-12119
             Project: Hadoop HDFS
          Issue Type: Bug
    Affects Versions: 2.6.0
            Reporter: liuyiyang


Sometimes the information "Number of Under-Replicated Blocks" shown on NameNode web UI is inconsistent with the "Under-replicated blocks" information shown in fsck report.

It's easy to reproduce such a case as follows:
1、In a cluster with DN1(rack0)、DN2(rack1) and DN3(rack2) which stores a lot of blocks, the replication factor is set to 2;
2、Re-allocate racks as DN1(rack0)、DN2(rack1) and DN3(rack1) ;
3、Restart HDFS daemons.
Then you can find inconsistent  "Number of Under-Replicated Blocks" on web ui and "Under-replicated blocks" in fsck result.

I dug into the source code and found that  "Number of Under-Replicated Blocks" on web ui consists of blocks that have less than target number of replicas and blocks that have the right number of replicas, but which the block manager felt were badly distributed.  In fsck result, "Under-replicated blocks" are the blocks that have less than target number of replicas, and "Mis-replicated blocks" are the blocks that are badly distributed.  So the Under-Replicated Blocks info on web UI and fsck result may be inconsistent.

Since Under-Replicated Blocks means higer missing risk for blocks, when threre is  no blocks that have less than target replicas but a lot of blocks that have the right number of blocks but are badlly distributed, the "Number of Under-Replicated Blocks" on web UI will be same as number of "Mis-replicated blocks", which is misleading for users. 
It would be clear to make "Number of Under-Replicated Blocks" on web UI be consistent with "Under-replicated blocks" in fsck result. 



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