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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Thu, 22 Mar 2007 20:12:32 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12483297
] 

Raghu Angadi commented on HADOOP-1134:
--------------------------------------


I think I need a little bit more clarification : There are multiple tasks:

Client :
--------
a)  It needs confidence DFS stores the data it writes and gets back the same data when it
reads.
      1) For client to be _absolutely_ sure of this, we need the current .crc files i.e. client
is assuming the responsibility of maintaining checksums
      2) current proposal slightly relaxes this and expects the clients to trust DFS to maintain
data to best of its ability and try client's best to verify it. ie, trusts that DN verifies
checksum that client calculates while writing, and maintains it after that etc.

If (2) is not acceptable, then probably we don't need block level checksums.


> 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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