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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12500467
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Doug Cutting commented on HADOOP-1134:
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Some comments on the design doc:
1. .meta seems like a more obvious extension than .mtd for metadata.
2. Re: "common header": should packets be length-prefixed? That might make
processing easier.
3. Re: "DATA_CHUNK": Unless I misunderstand, this only supports a single
checksum per chunk and a single chunk per packet. So, if we want to support
the current 512-byte checksums, this requires a packet every 512 bytes, which
seems small to me. I'd prefer something like:
<header, OP=WRITE, start, length, bytesPerChecksum, <data, checksum>*>
So, if length<=bytesPerChecksum, there's just a single checksum, as in your
proposal, but if length > bytesPerChecksum then there are multiple checksums in
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
> 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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