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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Tue, 20 Mar 2007 19:24:33 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12482494
] 

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

Regd Datanode upgrade : Online or offline?

Offline : When datanodes restart with the new version, they essentially go offline for couple
of hours (100+ GB)  come up with a new and shiny blocks.   
     Pros: 
           1) Simpler code. upgrade code does not need to be maintained in future versions.
Users with with very old dfs could upgrade iteratively.
            2) No change in block filenames or other maintenance is required.
     Cons: 
            1) Cluster would in accessible for couple of hours which implies lot of one time
work especially admins.
            2) Upgrade to future versions from current versions requires multiple upgrades.

Online: Datanode would immediately start serving existing data after restart. It will upgrade
the blocks in background.
       Pros:
            1) no extra downtime.
      Cons:
            1) Increases the code and this exra code would be spread around the datanode code.
            2) The code to handle mixed database may not be removed anytime in near future.

My (selfish) preference is to upgrade offline :). It does not seem unreasonable before 1.0
release.




> 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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