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 sync?

> 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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