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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12486481
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Doug Cutting commented on HADOOP-1134:
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> we should have some conventions like 'checksum of 0000 followed by some magic
> 8 bytes means checksum is incorrect' or some such thing
That assumes that the checksum is validated, doesn't it? And we're not yet
validating checksums except in the client, so I don't see where such an
encoding would be used. Right now we only need a way for the datanode to
indicate either (1) that there is a checksum, and here it is; or (b) there is
no checksum.
> if CRC file does not exist, it will be treated just like a corrupt block
The client might handle these cases differently. If the data does not match
the checksum, then odds are the data is invalid. However if the data has no
checksum then odds are good that the data is valid. So one might reasonably
configure the client to permit reading of data without checksums but to throw
exceptions for data whose checksum does not match the data.
> 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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