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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Tue, 27 Mar 2007 19:09:32 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12484548

Raghu Angadi commented on HADOOP-1134:

> It's worth noting that, if a .crc file cannot be found for a block, then the upgrade
should probably generate one (along with a warning). 
> So that, after the upgrade, all blocks should be guaranteed to have CRC files. We still
might fail softly when a CRC file is missing, 
> logging a warning and either generating CRCs on the fly or regenerating the CRC file.

I agree. We will generate local CRC file if we can not find ".crc" file data with a warning.
I think datanode should fail softly with a warning if CRC file is missing. I don't think it
is necessary to regenerate the crc file if one is missing outside upgrade mode.. it should
be normal error condition.

Thanks for feedback. I am preparing an HTML file and will include all the decisions made till
now and later comments for this jira. Will attach the HTML file here.

> 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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