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Date: Wed, 25 May 2011 01:24:47 +0000 (UTC)
From: "Todd Lipcon (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Updated] (HDFS-1994) Fix race conditions when running two
 rapidly checkpointing 2NNs
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     [ https://issues.apache.org/jira/browse/HDFS-1994?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Todd Lipcon updated HDFS-1994:
------------------------------

    Attachment: hdfs-1994.txt

Here's a preliminary patch. It needs some more unit tests that trigger the interleaving described above.

Doing the stress test described, it now runs a lot more stably, though there's still one unrelated issue in the 2NN (to be addressed later)

> Fix race conditions when running two rapidly checkpointing 2NNs
> ---------------------------------------------------------------
>
>                 Key: HDFS-1994
>                 URL: https://issues.apache.org/jira/browse/HDFS-1994
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: name-node
>    Affects Versions: Edit log branch (HDFS-1073)
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>             Fix For: Edit log branch (HDFS-1073)
>
>         Attachments: hdfs-1994.txt
>
>
> HDFS-1984 added the ability to run two secondary namenodes at the same time. However, there were two races I found when stress testing this (by running two NNs each checkpointing in a tight loop with no sleep):
> 1) the writing of the seen_txid file was not atomic, so it was at some points reading an empty file
> 2) it was possible for two checkpointers to try to take a checkpoint at the same transaction ID, which would cause the two image downloads to collide and fail

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