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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12500483
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Raghu Angadi commented on HADOOP-1134:
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Regd checksum calculation at the source, as we discussed on hadoop-dev, it
should ideally be solved by not buffering data at the higher level. There is no
reason to think higher level knows best what to buffer and how to buffer.
That leaves the problem with the validation while reading.
DistributedFileSystem does not a ChecksumFileSystem any more. Should it be?
Similarly once we make DistributedFileSystem not buffer any data, would that
address this issue?
This issue needs to be and will be addressed, especially since we know that
most often, memory is the culprit.
Regd sharing the code, except for the fact that both use CRC32 class, almost
everything is different about the implementation. Process of making these share
the code would result in quite a few changes to ChecksumFileSystem. May be that
should be a different Jira?
> 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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