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From "Hairong Kuang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Wed, 11 Jul 2007 00:30:04 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12511620
] 

Hairong Kuang commented on HADOOP-1134:
---------------------------------------

When I test Block-level crc upgrade, I set a file's replication factor to be 4. Afterwards
I manually corrupted 3 out of 4  replicas of the crc file and then upgraded it to block-level
crc dfs. Although there is one correct crc replica before upgrade, the current block upgrade
algothrim does not handle the case well and reading the file gets a ChecksumError after the
upgrade. 

It would be nice if block upgrade keeps all the crc replicas instead of taking the majority
replicas when there is a crc mismatch. For example, for data block A, if we have replicas
A1, A2, and A3, and crc replicas CRC1, CRC2, CRC3, then we can have Ai creates its meta file
by reading CRCi (i=1,2,3). To form a one-to-one mapping from replica Ai to CRCi, we can sort
all data replicas and sort all its crc replicas respectively. 

> 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: BlockLevelCrc-07032007.patch, BlockLevelCrc-07052007.patch, BlockLevelCrc-07062007.patch,
BlockLevelCrc-07062007.patch, BlockLevelCrc-07062007.patch, DfsBlockCrcDesign-05305007.htm,
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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