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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Mon, 09 Jul 2007 22:01:06 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12511272

Raghu Angadi commented on HADOOP-1134:

DFSOutputStream needs to have its own write() in any case, since it needs to decide how much
can be written to current block and write the rest of the data to next block. This is the
case even if block size a multiple of bytesPerChecksum. So DFSOutputStream needs to have a
'BlockWriter' class that subclasses OutputSummer (similar to BlockReader in DFSInputStream).

I was mistaken, this is not correct. Multiple blocks can be handled inside writeChunk(); May
be we should change writeChunk() also to pass in checksum, just like readChunk() takes checksum
in the same call.

> 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: BlockLevelCrc-07032007.patch, BlockLevelCrc-07052007.patch, BlockLevelCrc-07062007.patch,
BlockLevelCrc-07062007.patch, BlockLevelCrc-07062007.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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