[
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.
--
This message is automatically generated by JIRA.
-
You can reply to this email to add a comment to the issue online.
