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https://issues.apache.org/jira/browse/ZOOKEEPER-2872?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16122093#comment-16122093
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Hadoop QA commented on ZOOKEEPER-2872:
--------------------------------------

-1 overall.  GitHub Pull Request  Build
      

    +1 @author.  The patch does not contain any @author tags.

    +1 tests included.  The patch appears to include 6 new or modified tests.

    +1 javadoc.  The javadoc tool did not generate any warning messages.

    +1 javac.  The applied patch does not increase the total number of javac 
compiler warnings.

    +1 findbugs.  The patch does not introduce any new Findbugs (version 3.0.1) 
warnings.

    +1 release audit.  The applied patch does not increase the total number of 
release audit warnings.

    -1 core tests.  The patch failed core unit tests.

    +1 contrib tests.  The patch passed contrib unit tests.

Test results: 
https://builds.apache.org/job/PreCommit-ZOOKEEPER-github-pr-build/937//testReport/
Findbugs warnings: 
https://builds.apache.org/job/PreCommit-ZOOKEEPER-github-pr-build/937//artifact/trunk/build/test/findbugs/newPatchFindbugsWarnings.html
Console output: 
https://builds.apache.org/job/PreCommit-ZOOKEEPER-github-pr-build/937//console

This message is automatically generated.

> Interrupted snapshot sync causes data loss
> ------------------------------------------
>
>                 Key: ZOOKEEPER-2872
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2872
>             Project: ZooKeeper
>          Issue Type: Bug
>          Components: server
>    Affects Versions: 3.4.10, 3.5.3, 3.6.0
>            Reporter: Brian Nixon
>
> There is a way for observers to permanently lose data from their local data 
> tree while remaining members of good standing with the ensemble and 
> continuing to serve client traffic when the following chain of events occurs.
> 1. The observer dies in epoch N from machine failure.
> 2. The observer comes back up in epoch N+1 and requests a snapshot sync to 
> catch up.
> 3. The machine powers off before the snapshot is synced to disc and after 
> some txn's have been logged (depending on the OS, this can happen!).
> 4. The observer comes back a second time and replays its most recent snapshot 
> (epoch <= N) as well as the txn logs (epoch N+1). 
> 5. A diff sync is requested from the leader and the observer broadcasts 
> availability.
> In this scenario, any commits from epoch N that the observer did not receive 
> before it died the first time will never be exposed to the observer and no 
> part of the ensemble will complain. 
> This situation is not unique to observers and can happen to any learner. As a 
> simple fix, fsync-ing the snapshots received from the leader will avoid the 
> case of missing snapshots causing data loss.



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