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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-2872) Interrupted snapshot sync causes data loss
Date Sun, 13 Aug 2017 05:16:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-2872?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16124802#comment-16124802

ASF GitHub Bot commented on ZOOKEEPER-2872:

Github user hanm commented on the issue:

    I am now wondering why we should not fsync snapshot taking at all cases. It seems to be
a useful property to have for snapshot serialization, and will make code simpler. Any performance
considerations that lead to the conclusion of only applying fsync snapshot when it's a SNAP

> 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
> 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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