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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Fri, 01 Jun 2007 17:55:15 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12500783
] 

Raghu Angadi commented on HADOOP-1134:
--------------------------------------


>> only notable code replication I see is retry logic in FSInputChecker.readBuffer()
where SeekToNewSource() and reportChecksumFailure are executed
> Which is some of the most delicate code, that has taken several revisions to get to its
current level of correctness. In other words, logic that shouldn't be replicated if at all
possible.

Of course, code reuse is good. But in this case we need to write equally important tricky
and logic (in more than one place) to support sharing of another piece of tricky code. But
this is probably considered better since there is no code replication.

Also most Jira's filed for ChecksumFileSystem are to do with the complication of maintaining
two independent streams that are related to each other by offsets. Also each stream need to
be retried correctly  with different blocks. It is complecated but Block Level CRCs does not
have that problem since it only needs to deal with one stream and one type of retry. But new
Jira will be filed any way.




> 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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