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From "Ajay (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5711) Removing memory limitation of the Namenode by persisting Block - Block location mappings to disk.
Date Thu, 02 Jan 2014 21:33:51 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5711?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13860795#comment-13860795

Ajay commented on HDFS-5711:

Hi [~sureshms]

Rohan ( [~rohanpasalkar] ) and I ( [~Aj0Ay] ) intend to work on this  together.

To clean up the jira - our intention was not to tag along with the umbrella jira HDFS-2362
[https://issues.apache.org/jira/browse/HDFS-2362]. Please let us know if we should have a
single separate jira to track changes required to persist all the memory resident metadata
and then have subtasks for each metadata component? 

Our intention is to address the issue of persisting the block information (esp. since Lin
Xiao {intern at Hortonworks} had already worked on a prototype to persist the namespace information),
but to design the solution such that the persistent storage layer could be re-used to persist
other NN memory resident metadata. 

> Removing memory limitation of the Namenode by persisting Block - Block location mappings
to disk.
> -------------------------------------------------------------------------------------------------
>                 Key: HDFS-5711
>                 URL: https://issues.apache.org/jira/browse/HDFS-5711
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode
>            Reporter: Rohan Pasalkar
> This jira is to track changes to be made to remove HDFS name-node memory limitation to
hold block - block location mappings.
> It is a known fact that the single Name-node architecture of HDFS has scalability limits.
The HDFS federation project alleviates this problem by using horizontal scaling. This helps
increase the throughput of metadata operation and also the amount of data that can be stored
in a Hadoop cluster.
> The Name-node stores all the filesystem metadata in memory (even in the federated architecture),
> Name-node design can be enhanced by persisting part of the metadata onto secondary storage
and retaining 
> the popular or recently accessed metadata information in main memory. This design can
benefit a HDFS deployment
> which doesn't use federation but needs to store a large number of files or large number
of blocks. Lin Xiao from Hortonworks attempted a similar
> project [1] in the Summer of 2013. They used LevelDB to persist the Namespace information
(i.e file and directory inode information).
> A patch with this change is yet to be submitted to code base. We also intend to use LevelDB
to persist metadata, and plan to 
> provide a complete solution, by not just persisting  the Namespace information but also
the Blocks Map onto secondary storage. 
> We did implement the basic prototype which stores the block-block location mapping metadata
to the persistent key-value store i.e. levelDB. Prototype also maintains the in-memory cache
of the recently used block-block location mappings metadata. 
> References:
> [1] Lin Xiao, Hortonworks, Removing Name-node’s memory limitation, http://www.slideshare.net/ydn/hadoop-meetup-hug-august-2013-removing-the-namenodes-memory-limitation

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