[jira] [Commented] (HDFS-5434) Write resiliency for replica count 1

["Arpit Agarwal (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-5434?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13847967#comment-13847967 ] 

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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