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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1134) Block level CRCs in HDFS
Date Wed, 21 Mar 2007 18:57:32 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12482872
] 

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


Correctness is a big advantage of using existing CRCs. Still, we could choose to 64k checksums
during upgrade, which implies reading of of all the blocks and associated increase in downtime.
Whether to recreate checksums or not depends on how far off is 512 byte CRCs is from our (gu)estimated
optimal value. Another option to get more trust after to upgrade is to compare checksums of
replicas and choose the majority. For current design we can  assume we compare with the old
CRCs.

If a client asks for 2k data, we are saying we should send 64k (or 128k) where 2k is located
to client. Why not send only 2k and send newly calculated CRC for 2k? Datanode still verifies
all the 64k blocks involved in the read but recalculates. One argument against this is that
this is a weaker guarantee than sending full blocks for verification. But datanode will calculate
the new CRC right next to where the on disk CRC is verified thus minimizing some other corruptions.
I feel this compromise is probably worth it. But I guess many will disagree. Requiring whole
blocks will also increase overhead when support appends in future.

How do we benchmark for good CRC-chunk size? It heavily depends on work load. I will find
more about a typical MapReduce load.





> 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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