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From "Sameer Paranjpye (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Fri, 30 Mar 2007 18:45:25 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12485605

Sameer Paranjpye commented on HADOOP-1134:

> But actually, now that I think about it, if we're primarily not validating checksums
against the data, but rather comparing all the checksums for a block, then locality may not
> worthwhile. In that case we'd want a temporary datanode extension that permits writing
the checksum file for a block. Then the updater map task can read through all copies of 
> the checksum file, construct the best possible checksum for each block, then send these
to datanodes. So, in aggregate, 6% of the filesystem would cross the wire during the 
> upgrade. Could that work? 

If the client is going to send CRCs to Datanodes then we can have each split be a filename
or a list of filenames, this will work fine if we don't want to validate data. If we want
validation a map task can be scheduled local to 1 instance of most or all blocks of a file
i.e. scheduled on the node where the file was generated, it can validate local data, fall
back to remote data if local validation fails, then write checksums to all the block instances.

How do we manage blocks that are missing during the upgrade? There are 3 cases really:
- Blocks that are entirely missing i.e. no instances are available
- Blocks that have some instances missing
- Blocks whose checksums are missing

> 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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