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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12482958
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Raghu Angadi commented on HADOOP-1134:
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I will summarize various points discussed here later today.
Regd informing namenode(NN) about corrupted blocks :
1) Initially we will do same thing DFSClient does now : tells NN to delete
DN:Block. NN schedules it to be deleted if there are more than one replicas.
Ideally we want the new replica to be created before deleting the known
corrupted blocks.
2) Initially DN detects corrupted blocks when they are read for any reason.
Pretty soon we will add periodic scan of blocks on datanode. Periodic checker
should throttle itself based on how busy the disks are.
3) Later, if required, we could add an option such that DFSClient can read even
if checksum fails (with failure promptly noted). For now, it will always be an
error.
> 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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