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https://issues.apache.org/jira/browse/HDFS-5318?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13906153#comment-13906153
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Hadoop QA commented on HDFS-5318:
---------------------------------
{color:green}+1 overall{color}. Here are the results of testing the latest
attachment
http://issues.apache.org/jira/secure/attachment/12629857/HDFS-5318-trunk-c.patch
against trunk revision .
{color:green}+1 @author{color}. The patch does not contain any @author
tags.
{color:green}+1 tests included{color}. The patch appears to include 4 new
or modified test files.
{color:green}+1 javac{color}. The applied patch does not increase the
total number of javac compiler warnings.
{color:green}+1 javadoc{color}. There were no new javadoc warning messages.
{color:green}+1 eclipse:eclipse{color}. The patch built with
eclipse:eclipse.
{color:green}+1 findbugs{color}. The patch does not introduce any new
Findbugs (version 1.3.9) warnings.
{color:green}+1 release audit{color}. The applied patch does not increase
the total number of release audit warnings.
{color:green}+1 core tests{color}. The patch passed unit tests in
hadoop-common-project/hadoop-common hadoop-hdfs-project/hadoop-hdfs.
{color:green}+1 contrib tests{color}. The patch passed contrib unit tests.
Test results:
https://builds.apache.org/job/PreCommit-HDFS-Build/6175//testReport/
Console output: https://builds.apache.org/job/PreCommit-HDFS-Build/6175//console
This message is automatically generated.
> Support read-only and read-write paths to shared replicas
> ---------------------------------------------------------
>
> Key: HDFS-5318
> URL: https://issues.apache.org/jira/browse/HDFS-5318
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Affects Versions: 2.3.0
> Reporter: Eric Sirianni
> Attachments: HDFS-5318-trunk-c.patch, HDFS-5318-trunk.patch,
> HDFS-5318-trunkb.patch, HDFS-5318.patch, HDFS-5318a-branch-2.patch,
> HDFS-5318b-branch-2.patch, HDFS-5318c-branch-2.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)
> ** → 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)
> ** → 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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