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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12482279
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Raghu Angadi commented on HADOOP-1134:
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This is surely going to be sub block. For every 64k bytes or so. "Block level
CRCs" is probably misleading title. Suggestions are welcome.
Inline CRCs sounds good to me:
1) Not sure of any disadvantages of storing inline since how blocks are stored
is totally internal to Datanodes.
2) It does save a seek if we are reading small chunks of data. But in many
cases, we would be reading many megabytes serially. This way the seeks saved
may not be great. But every seek saved helps.
3) It save files opened and closed as well for each block accessed.
4) It also matches how I was thinking of sending CRCs to client and peers (
inline with on the same data connection instead of a separate channel ).
> 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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