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https://issues.apache.org/jira/browse/HDFS-5434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13847967#comment-13847967
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Arpit Agarwal commented on HDFS-5434:
-------------------------------------
Eric, what is the advantage of retaining only one replica? Unless it is
temporary data that can be easily regenerated, having one replica guarantees
data loss in quick order. e.g. on a 5000-disk cluster with an AFR of 4% you'd
expect at least one disk failure every other day. This setting sounds somewhat
dangerous setting as it could give users a false sense of reliability.
> 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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