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Arpit Agarwal commented on HDFS-5318: ------------------------------------- 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*. -- This message was sent by Atlassian JIRA (v6.1.5#6160)