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From "Eric Sirianni (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HDFS-5434) Write resiliency for replica count 1
Date Mon, 28 Oct 2013 14:07:33 GMT

     [ https://issues.apache.org/jira/browse/HDFS-5434?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Eric Sirianni updated HDFS-5434:
--------------------------------

    Description: 
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 other 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}


  was:
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 other 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}



> 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 other 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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