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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3810) NameNode seems unstable on a cluster with little space left
Date Thu, 20 Nov 2008 23:58:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3810?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12649545#action_12649545

Raghu Angadi commented on HADOOP-3810:

> Once I have more details about a NameNode that was in this state

Still not sure what is actually affecting NN badly. The solution in description of the jira
assumes that it is the excessive CPU for allocating the blocks that is causing this problem.
But on a recent incident, ti does not look like there are many blocks being written. In fact
at one point everything pretty much comes to halt. Don't have gc.log to see if this was a

I will attach couple of simon graphs that show what was going on during this time.

> NameNode seems unstable on a cluster with little space left
> -----------------------------------------------------------
>                 Key: HADOOP-3810
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3810
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>    Affects Versions: 0.17.1
>            Reporter: Raghu Angadi
>            Assignee: Raghu Angadi
> NameNode seems not very responsive and unstable when the cluster has very little space
left. The clients timeout. The main problem is that it is not clear to the user what is going
on. Once I have more details about a NameNode that was in this state, I will fill in here.
> If there is not enough space left on a cluster, it is ok for clients to receive something
like "DiskOutOfSpace" exception. 
> Right now it looks like NameNode tries too hard find a node with any space left and ends
up being slow to respond to clients. If the CPU taken by chooseTarger() is the main cause,
there are two possible fixes :
> # chooseTarget() iterates and takes quite a bit of CPU for allocating datanodes. Usually
this not much of a problem. It takes even more cpu when it needs to search multiple racks
for a datanode. We could probably reduce some CPU for these searches. The benefit should be
> # Once NameNode can not find any datanode that has space on a rack, it could mark the
rack as "full" and skip searching the rack for next one minute or so. This flag gets cleared
after a minute or if any new node is added to the rack.
> #* Of course, this might not be optimal w.r.t disk space usage.. but only for a short
duration. Once a cluster is mostly full, the user does expect errors.
> #* On the flip side, this fix does not require extremely CPU optimized version of chooseTarget().

> #* I think it is reasonable for NameNode to throw DiskOutOfSpace exception, even though
it could have found space if it searched much more extensively.
> ---
> edit : minor changes

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