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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Created: (HADOOP-1134) Block level CRCs in HDFS
Date Mon, 19 Mar 2007 22:28:32 GMT
Block level CRCs in HDFS

                 Key: HADOOP-1134
                 URL: https://issues.apache.org/jira/browse/HADOOP-1134
             Project: Hadoop
          Issue Type: New Feature
          Components: dfs
            Reporter: Raghu Angadi
         Assigned To: Raghu Angadi

Currently CRCs are handled at FileSystem level and are transparent to core HDFS. See recent
improvement HADOOP-928 ( that can add checksums to a given filesystem ) regd more about it.
Though this served us well there a few disadvantages :

1) This doubles namespace in HDFS ( or other filesystem implementations ). In many cases,
it nearly doubles the number of blocks. Taking namenode out of CRCs would nearly double namespace
performance both in terms of CPU and memory.

2) Since CRCs are transparent to HDFS, it can not actively detect corrupted blocks. With block
level CRCs, Datanode can periodically verify the checksums and report corruptions to namnode
such that name replicas can be created.

We propose to have CRCs maintained for all HDFS data in much the same way as in GFS. I will
update the jira with detailed requirements and design. This will include same guarantees provided
by current implementation and will include a upgrade of current data.


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