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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12500795
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Raghu Angadi commented on HADOOP-1134:
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bq. It's a minor point, but if we write async-io daemons for this protocol then
the easier it is to parse the total packet length the easier it will be to
write these daemons. So placing the total packet length in a fixed position at
the front of the packet so that it may be generically accessed without having
to determine what kind of a packet it is, will simplify things.
Agreed. Btw, these are not really packets in any sense. These are streams with
typical lengths of 100s of MB. Of course each DATA_CHUNK is like packet and it
does include a length. Though using Vints negates simplicity of Async reading.
In fact, knowing lengths for DATA_CHUNK helps even non-async processing and new
code uses the length heavily.
We might even get rid of Initial length (or set it to -1) for OP_WRITE_BLOCK if
we want to move to streaming block to DN in parallel to client's writes
(instead of transferring the whole block inside endBlock()).
> 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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