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From "Eric Sirianni (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5434) Write resiliency for replica count 1
Date Fri, 20 Dec 2013 20:14:10 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13854521#comment-13854521

Eric Sirianni commented on HDFS-5434:

bq. But the propose solution should work in that case as well right?
OK - I see the misunderstanding.  I interpreted above comment to mean that "*this* proposed
solution *will* work in the append case" (which it won't :))

bq.  the proposed solution is to use two replicas. That does not solve the append scenario.
My concern is, we should think comprehensively about all the issues and solutions.
I agree.  Having a pipeline with length >1 in the append case with repcount=1 seems to
intrinsically require having that single replica be shared, right?  To solve the initial write
in the same manner, we would need the initial write pipeline to also be setup in a "shared
storage aware" manner.  This might be possible in a {{BlockPlacementPolicy}} plugin by using
{{NodeGroup}}s to demarcate groups of DataNodes that share storage.

> Write resiliency for replica count 1
> ------------------------------------
>                 Key: HDFS-5434
>                 URL: https://issues.apache.org/jira/browse/HDFS-5434
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: namenode
>    Affects Versions: 2.2.0
>            Reporter: Buddy
>            Priority: Minor
>         Attachments: BlockPlacementPolicyMinPipelineSize.java, BlockPlacementPolicyMinPipelineSizeWithNodeGroup.java,
> If a file has a replica count of one, the HDFS client is exposed to write failures if
the data node fails during a write. With a pipeline of size of one, no recovery is possible
if the sole data node dies.
> A simple fix is to force a minimum pipeline size of 2, while leaving the replication
count as 1. The implementation for this is fairly non-invasive.
> Although the replica count is one, the block will be written to two data nodes instead
of one. If one of the data nodes fails during the write, normal pipeline recovery will ensure
that the write succeeds to the surviving data node.
> The existing code in the name node will prune the extra replica when it receives the
block received reports for the finalized block from both data nodes. This results in the intended
replica count of one for the block.
> This behavior should be controlled by a configuration option such as {{dfs.namenode.minPipelineSize}}.
> This behavior can be implemented in {{FSNameSystem.getAdditionalBlock()}} by ensuring
that the pipeline size passed to {{BlockPlacementPolicy.chooseTarget()}} in the replication
parameter is:
> {code}
> max(replication, ${dfs.namenode.minPipelineSize})
> {code}

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