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https://issues.apache.org/jira/browse/HDFS-5318?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13876006#comment-13876006
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Arpit Agarwal commented on HDFS-5318:
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Hi Eric, initial feedback below.
# BlockManager.java#chooseSourceDatanode
#* Fix {{assert containingLiveReplicasNodes.size() ==
numReplicas.liveReplicas()}} in {{BlockManager#dumpBlockMeta}}.
# MiniDFSCluster#startDataNodes
#* Document what the new parameter {{dnConfOverlays}} is doing.
# General:
#* I think it would be a good idea to rename READ_ONLY to READ_ONLY_SHARED
everywhere (including comments) and document the meaning. What do you think?
Also we can document that replicas on READ_ONLY_SHARED storages are not counted
towards live replicas.
#* Remove lines with whitespace-only changes.
# Testing
#* Add test case for replication factor = 1, bring down read-write replica.
Check metric counts. Is read-only replica used to replicate to a read-write
storage?
#* rename function {{#test()}} to something better e.g. {{testReplicaCounting}}
I am not sure if this change affects lease recovery. It would be good if
someone familiar with how lease recovery works can take a quick look at the
approach. We don't want read-only replicas to participate in recovery. What if
all read-write replicas are unavailable?
> 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: 2.4.0
> Reporter: Eric Sirianni
> Attachments: HDFS-5318.patch, HDFS-5318a-branch-2.patch,
> HDFS-5318b-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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