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https://issues.apache.org/jira/browse/ZOOKEEPER-2872?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16122076#comment-16122076
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ASF GitHub Bot commented on ZOOKEEPER-2872:
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GitHub user enixon opened a pull request:
https://github.com/apache/zookeeper/pull/333
ZOOKEEPER-2872: Interrupted snapshot sync causes data loss
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/enixon/zookeeper snap-sync
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/zookeeper/pull/333.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #333
----
commit 39bd1a3eb9171a014845fff97648341cbfb40a11
Author: Brian Nixon <ni...@fb.com>
Date: 2017-08-01T20:25:51Z
ZOOKEEPER-2872: Interrupted snapshot sync causes data loss
----
> 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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