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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12500516
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Doug Cutting commented on HADOOP-1134:
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> How do we implementation this? My preferred method is to FS implementation to
> do the the buffering.
Yes, FileSystem#open(Path, bufferSize) should return a buffered stream, and
that method is abstract, so buffering is implemented by the FileSystem. But
two FileSystem implementations might reasonably share some of their buffering
code. In this case, I think there's lots to share between ChecksumFileSystem
and DistributedFileSystem. We've found a number of subtle bugs in the
checksumming code, and there may be more. Two separate implementations will
double the bugs.
I've submitted a separate issue to fix the buffering issue, HADOOP-1450. It's
simple & will probably be committed long before this issue is complete.
> 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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