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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Thu, 21 Jun 2007 23:14:26 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12507066
] 

Konstantin Shvachko commented on HADOOP-1134:
---------------------------------------------

> A detailed report iterates through all the blocks and checks how many blocks belong to
following categories

# The categories you define do not add up to the complete number of blocks. If I counted blocks
I'd define
the following categories:
-- Fully replicated blocks (r >= target replication);
-- minimally-replicated blocks (minReplicas <= r < target replication);
-- under-replicated blocks (r < minReplicas)
# But I'd rather report it in more general terms like an overall percentage of the upgrade
actually completed.
For the CRC upgrade it is the percentage of blocks that have at least the minimal # of replicas,
other distr upgrades can measure their completeness in different terms.

> 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, readBuffer.java,
readBuffer.java
>
>
> 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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