hadoop-hdfs-issues mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Eric Sirianni (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5318) Pluggable interface for replica counting
Date Fri, 03 Jan 2014 19:11:52 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5318?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13861798#comment-13861798
] 

Eric Sirianni commented on HDFS-5318:
-------------------------------------

Arpit - have you had a chance to think any more on this?

I agree that using "writability" of a replica as a proxy for deducing whether or not that
replica is shared is a bit awkward.  However, I understand yours and Suresh's points and concerns
raised above. 

Should we proceed with the R/O approach as you outlined above and table the goal of more robustly
introducing the concept of "logical" vs. "physical" replicas until the future?

> 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*.



--
This message was sent by Atlassian JIRA
(v6.1.5#6160)

Mime
View raw message