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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 Mon, 24 Nov 2008 23:25:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3810?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12650379#action_12650379
] 

Raghu Angadi commented on HADOOP-3810:
--------------------------------------

The recent suspected incidence of this problem showed that NN failed to find a node on some
racks only around 2000 attempts. That would account for no more than a few seconds of CPU
time. I suspect the fix I proposed in the description does not actually fix or reduce this
problem. 

> 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
>         Attachments: simon-namenode.PNG
>
>
> 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
measurable.
> # 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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