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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12511272
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Raghu Angadi commented on HADOOP-1134:
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{quote}
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).
{quote}
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, readBuffer.java
>
>
> 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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