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From "Sameer Paranjpye (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Wed, 28 Mar 2007 19:50:25 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12484974
] 

Sameer Paranjpye commented on HADOOP-1134:
------------------------------------------


> If we cannot get old CRC data for any reason, we will generate one based on the local
data (which could be wrong). There are two options to validate upgraded data (for 
> simplicity all the details and error conditions are not explained) :
> 1) use old CRCs (Doug's choice)
> 2) check CRC of each replica and choose the majority (Sameer's choice)
> 3) Combination of (1) and (2). i.e. use (2) if (1) fails etc. This option is proposed
only now.

Even if we *can* get the old CRC data, how do we know that it is not corrupt? There are 3
copies of each CRC file, one or more of these could be corrupt. We need some way to ensure
that we're copying correct checksum data to the Datanode. As I said before, we can do this
by comparing copies of the existing CRC data against each other and electing a set of authorities
OR by validating checksum data that we pull against the local blocks.

> 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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