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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-5434) Write resiliency for replica count 1
Date Fri, 01 Nov 2013 21:07:17 GMT

    [ https://issues.apache.org/jira/browse/HDFS-5434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13811661#comment-13811661
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Colin Patrick McCabe commented on HDFS-5434:
--------------------------------------------

I think having a client-side config is reasonable.  We need to document well the scenarios
when this would be useful, and I definitely wouldn't want it to be on by default, but in some
use cases it makes sense.

> 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
>
> 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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