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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-1134) Block level CRCs in HDFS
Date Wed, 30 May 2007 16:48:15 GMT

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

Raghu Angadi updated HADOOP-1134:
---------------------------------

    Attachment: bc-no-upgrade-05302007.patch

Current patch is attached. This is a VERY EXPERIMENTAL patch. A few notes:
 
 # Applies to trunk as of 05/30/07.
 # This will not start on existing DFS. You need first format with this patch.
 # DFS is fully functional and is not expected be any slower than current DFS. But this in
no way close to being final patch.
 # There is a bug chunk of Upgrade related in DataNode.java and a little bit in FSNameSystem.java
but it is not currently excuted.
 # {{DistributedFileSystem}} in trunk becomes {{ChecsumDistributedFileSystem}} with the patch.
{{DistributeFileSystem}} in the patch does not create {{.crc}} files.
 # Core functionality and protocol is described in the html file attached with previous comment.
 # The important changes are in DFSClient.java and DataNode.java
 #  DFSClient.BlockReader class handles reading from a datanode and unpacks the data.
 # There are quite a few {{XXX}} comments and will be handled in later patches.


> 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
>            Assignee: Raghu Angadi
>         Attachments: bc-no-upgrade-05302007.patch, DfsBlockCrcDesign-05305007.htm
>
>
> 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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