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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1508) Ability to do savenamespace without being in safemode
Date Wed, 01 Dec 2010 11:23:11 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1508?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12965663#action_12965663
] 

dhruba borthakur commented on HDFS-1508:
----------------------------------------

Thanks Sanjay and Konstantin for looking at this one.

First the use case: putting the namenode in safemode causes existing applications to fail.
This is a severe problem for us. If you are using hdfs for running map-reduce jobs, then putting
the namenode is safemode means that tasks fail immediately. A reduce task that has been running
for a long long time will fail and has to start all over again. If you are running hbase on
hdfs, then hundreds of hbase region servers will die when the namenode goes into safemode.

@Sanjay: I cluster that runs hbase typically has very few files, less than  100K files. It
takes a few seconds to run the savenamespace command. I can generalize: if a user is running
hbase on hdfs, then it makes more sense to make savenamespace wait for a few seconds (via
the read/write lock) rather than writing special case code in the hbase region servers to
handle SafeModeException. As far as backward compatibility is concerned,  i can add a "-f"
option to indicate "do the savenamespace even if namenode is not in safemode", but still think
that this is an option that everybody will use.

Can one of you explain why we always required savenamespace to have namenode is safemode?
isn't it always better to stall the workload rather to fail the workload?

@Konstantin: can you pl explain the precise problem you have in mind? The rollFSImage() call
acquires the FSNamesystem writelock, so it cannot race with saveNamespace. Moreover, saveNamespace
truncates the edits log and removes edits.new while rollFSImage will exit out if it does not
find edits.new.






> Ability to do savenamespace without being in safemode
> -----------------------------------------------------
>
>                 Key: HDFS-1508
>                 URL: https://issues.apache.org/jira/browse/HDFS-1508
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>         Attachments: savenamespaceWithoutSafemode.txt
>
>
> In the current code, the administrator can run savenamespace only after putting the namenode
in safemode. This means that applications that are writing to HDFS encounters errors because
the NN is in safemode. We would like to allow saveNamespace even when the namenode is not
in safemode.
> The savenamespace command already acquires the FSNamesystem writelock. There is no need
to require that the namenode is in safemode too.

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