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From "Arpit Agarwal (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5318) Pluggable interface for replica counting
Date Thu, 19 Dec 2013 18:18:08 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5318?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13853093#comment-13853093
] 

Arpit Agarwal commented on HDFS-5318:
-------------------------------------

Thanks for posting a design doc. I will take a look at it today.

> Pluggable interface for replica counting
> ----------------------------------------
>
>                 Key: HDFS-5318
>                 URL: https://issues.apache.org/jira/browse/HDFS-5318
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode
>    Affects Versions: 2.4.0
>            Reporter: Eric Sirianni
>         Attachments: HDFS-5318.patch, hdfs-5318.pdf
>
>
> There are several use cases for using shared-storage for datanode block storage in an
HDFS environment (storing cold blocks on a NAS device, Amazon S3, etc.).
> With shared-storage, there is a distinction between:
> # a distinct physical copy of a block
> # an access-path to that block via a datanode.  
> A single 'replication count' metric cannot accurately capture both aspects.  However,
for most of the current uses of 'replication count' in the Namenode, the "number of physical
copies" aspect seems to be the appropriate semantic.
> I propose altering the replication counting algorithm in the Namenode to accurately infer
distinct physical copies in a shared storage environment.  With HDFS-5115, a {{StorageID}}
is a UUID.  I propose associating some minor additional semantics to the {{StorageID}} - namely
that multiple datanodes attaching to the same physical shared storage pool should report the
same {{StorageID}} for that pool.  A minor modification would be required in the DataNode
to enable the generation of {{StorageID}} s to be pluggable behind the {{FsDatasetSpi}} interface.
 
> With those semantics in place, the number of physical copies of a block in a shared storage
environment can be calculated as the number of _distinct_ {{StorageID}} s associated with
that block.
> Consider the following combinations for two {{(DataNode ID, Storage ID)}} pairs {{(DN_A,
S_A) (DN_B, S_B)}} for a given block B:
> * {{DN_A != DN_B && S_A != S_B}} - *different* access paths to *different* physical
replicas (i.e. the traditional HDFS case with local disks)
> ** &rarr; Block B has {{ReplicationCount == 2}}
> * {{DN_A != DN_B && S_A == S_B}} - *different* access paths to the *same* physical
replica (e.g. HDFS datanodes mounting the same NAS share)
> ** &rarr; Block B has {{ReplicationCount == 1}}
> For example, if block B has the following location tuples:
> * {{DN_1, STORAGE_A}}
> * {{DN_2, STORAGE_A}}
> * {{DN_3, STORAGE_B}}
> * {{DN_4, STORAGE_B}},
> the effect of this proposed change would be to calculate the replication factor in the
namenode as *2* instead of *4*.



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