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https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12485308
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Sameer Paranjpye commented on HADOOP-1134:
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> The same way we do today: we don't.
When the HDFS client encounters a checksum error, it doesn't know whether it is
the data or the checksum that is corrupt. But it takes care to switch both the
data block and the checksum block that it is reading. So while we don't *know*
whether the checksum is corrupt, we certainly don't assume that it's not.
Computing CRCs is cheap, every job that we run computes them on input and on
output, and it hasn't been a burden so far. We can benchmark how long it takes
to compute checksums for a block, but I doubt it would add significantly to the
time for upgrade.
> This patch should confine itself to removing checksums from the HDFS
> namespace
Perhaps, but even so, the removal should be done correctly. The system as it
exists today deals with the possibility of checksum corruption, it should do so
during and after the upgrade. Not doing so would be a regression.
> 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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