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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-4663) Datanode should delete files under tmp when upgraded from 0.17
Date Wed, 21 Jan 2009 07:24:59 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-4663?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12665744#action_12665744
] 

dhruba borthakur commented on HADOOP-4663:
------------------------------------------

The append design explicitly states that the system should make every effort to persist data
written to a file even is sync is not invoked. "sync" has some cost associated with it, and
an application might not want to incur the cost at every write, but t would definitely like
HDFS to make the best effort to persist that data it has written.

> it turned to be error-prone

The design is not error prone, it was the implementation that had some shortcomings. In particular,
blocks that are part of replication requests should not have been promoted. This is fixed
by this patch. If you are aware of other bugs in this area that can cause problems, it would
be really nice if you can list it out or create a unit test to trigger that bug. I understand
that the "append" protocol is complex, but when implemented correctly should be fool-proof.


>it adds performance overhead, when sync or append is not used.

are you concerned that renaming the block file from the blocksBeingWritten directory to the
main block directory adds overhead to the write-pipeline? 

> Datanode should delete files under tmp when upgraded from 0.17
> --------------------------------------------------------------
>
>                 Key: HADOOP-4663
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4663
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: dfs
>    Affects Versions: 0.18.0
>            Reporter: Raghu Angadi
>            Assignee: dhruba borthakur
>            Priority: Blocker
>             Fix For: 0.19.1
>
>         Attachments: deleteTmp.patch, deleteTmp2.patch, deleteTmp_0.18.patch, handleTmp1.patch
>
>
> Before 0.18, when Datanode restarts, it deletes files under data-dir/tmp  directory since
these files are not valid anymore. But in 0.18 it moves these files to normal directory incorrectly
making them valid blocks. One of the following would work :
> - remove the tmp files during upgrade, or
> - if the files under /tmp are in pre-18 format (i.e. no generation), delete them.
> Currently effect of this bug is that, these files end up failing block verification and
eventually get deleted. But cause incorrect over-replication at the namenode before that.
> Also it looks like our policy regd treating files under tmp needs to be defined better.
Right now there are probably one or two more bugs with it. Dhruba, please file them if you
rememeber.

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