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From "Robert Chansler (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5034) NameNode should send both both replication and deletion requests to DataNode in one reply to a heartbeat
Date Fri, 16 Jan 2009 02:04:59 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5034?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12664368#action_12664368
] 

Robert Chansler commented on HADOOP-5034:
-----------------------------------------

I first thought that just switching the priority to deletions rather than replications would
be satisfactory, but Hairong explained that since processing is time-sliced, if the higher
priority task occurs at the very modest rate of once per heartbeat, the lower priority task
will be starved. Starving deletions can make replication impossible. The reverse is not true,
but it is difficult to make the statistical argument that starving replications for a while
is OK. You can't starve replication too long by doing deletions as there are only so many
replicas to delete, but is it OK for replications to wait a minute? An hour? It is perhaps
best to just follow Hairong's suggestion.

> NameNode should send both both replication and deletion requests to DataNode in one reply
to a heartbeat
> --------------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-5034
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5034
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: dfs
>            Reporter: Hairong Kuang
>             Fix For: 0.21.0
>
>
> Currently NameNode favors block replication requests over deletion requests. On reply
to a heartbeat, NameNode does not send a block deletion request unless there is no block replication
request. 
> This brings a problem when a near-full cluster loses a bunch of DataNodes. In react to
the DataNode loss, NameNode starts to replicate blocks. However, replication takes a lot of
cpu and a lot of replications fail because of the lack of disk space. So the administrator
tries to delete some DFS files to free up space. However, block deletion requests get delayed
for very long time because it takes a long time to drain the block replication requests for
most DataNodes.
> I'd like to propose to let NameNode to send both replication requests and deletion requests
to DataNodes in one reply to a heartbeat. This also implies that the replication monitor should
schedule both replication and deletion work in one iteration.

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