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From "Eli Collins (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2936) Provide a better way to specify a HDFS-wide minimum replication requirement
Date Wed, 16 May 2012 20:37:06 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2936?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13277097#comment-13277097
] 

Eli Collins commented on HDFS-2936:
-----------------------------------

Why not just check the replication level at file creation/setrep and fail if it's lower than
the new configured value? No BlockManager changes etc.  This would require the user setrep
all existing files to the new minimum replication level when they set it, but that seems acceptable.
                
> Provide a better way to specify a HDFS-wide minimum replication requirement
> ---------------------------------------------------------------------------
>
>                 Key: HDFS-2936
>                 URL: https://issues.apache.org/jira/browse/HDFS-2936
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>    Affects Versions: 0.23.0
>            Reporter: Harsh J
>            Assignee: Harsh J
>         Attachments: HDFS-2936.patch
>
>
> Currently, if an admin would like to enforce a replication factor for all files on his
HDFS, he does not have a way. He may arguably set dfs.replication.min but that is a very hard
guarantee and if the pipeline can't afford that number for some reason/failure, the close()
does not succeed on the file being written and leads to several issues.
> After discussing with Todd, we feel it would make sense to introduce a second config
(which is ${dfs.replication.min} by default) which would act as a minimum specified replication
for files. This is different than dfs.replication.min which also ensures that many replicas
are recorded before completeFile() returns... perhaps something like ${dfs.replication.min.user}.
We can leave dfs.replication.min alone for hard-guarantees and add ${dfs.replication.min.for.block.completion}
which could be left at 1 even if dfs.replication.min is >1, and let files complete normally
but not be of a low replication factor (so can be monitored and accounted-for later).
> I'm prefering the second option myself. Will post a patch with tests soon.

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