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From "Dmitry Konstantinov (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ZOOKEEPER-3408) Improve information about risks for forceSync config option
Date Mon, 03 Jun 2019 14:20:00 GMT

    [ https://issues.apache.org/jira/browse/ZOOKEEPER-3408?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16854641#comment-16854641

Dmitry Konstantinov commented on ZOOKEEPER-3408:

Thank you for the link.
 Let's say we have 5 nodes in ZK ensemble with IDs: 1, 2, 3, 4, 5. 
 1 - the current leader.
 So, we are speaking about the following combination of multiple problems:
 1) Network or ZK node delays happened for some 2 ZK nodes (let's say 4 and 5), so last transactions
are not propagated to them.
 2) Then the current leader (1) is failed before an actual write of it's transaction to disk
and restarted.
 2) Then during the leader re-election due to a network partition or target ZK node delays
none of the ZK nodes with the latest transaction info (nodes: 2,3) were participated, so nodes
1, 4, 5 have elected a new leader which is not aware about the last transactions.

Did I get your case correctly?

> Improve information about risks for forceSync config option
> -----------------------------------------------------------
>                 Key: ZOOKEEPER-3408
>                 URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3408
>             Project: ZooKeeper
>          Issue Type: Improvement
>          Components: documentation
>            Reporter: Dmitry Konstantinov
>            Priority: Minor
> [https://zookeeper.apache.org/doc/r3.5.5/zookeeperAdmin.html#Unsafe+Options]
> {quote}
> The following options can be useful, but be careful when you use them. The risk of each
is explained along with the explanation of what the variable does.
> {quote}
> {quote}
> _forceSync_ : (Java system property: *zookeeper.forceSync*) Requires updates to be
synced to media of the transaction log before finishing processing the update. If this option
is set to no, ZooKeeper will not require updates to be synced to the media.
> {quote}
> The risks for this option are not very clear. Does it only the risk of loss some recent
committed transactions if all Zookeeper instances from an ensemble are crashed and restarted
at almost the same time or some other problems are also possible? 

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