Stig, Il giorno lun 11 gen 2021 alle ore 11:37 Stig Døssing < generalbas....@gmail.com> ha scritto:
> Sorry, I meant to say 3.5 introduced a check that snaphots exist, not that > the snapshot files don't exist in 3.4. > > On 2021/01/11 10:13:34, Stig D��ssing <generalbas....@gmail.com> wrote: > > Hi, > > > > Zookeeper 3.5 introduced snapshot files, which did not exist in 3.4. 3.5 > won't start if data is present, and there is no snapshot file. > > > > https://issues.apache.org/jira/browse/ZOOKEEPER-3056 added an option to > disable this check, to enable migration from 3.4 clusters. The workaround > before then was to add an empty snapshot file to the dataDir. > > > > As far as I can tell, the intended method of upgrading from 3.4 is to > add snapshot.trust.empty=true to the Zookeeper configuration, upgrade to > 3.5.x, and remove the snapshot.trust.empty property once snapshots exist on > all nodes. > > > > Sadly this method turns out to be inconvenient, as upgraded nodes will > not write snapshots immediately. See > https://issues.apache.org/jira/browse/ZOOKEEPER-3781?focusedCommentId=17261317&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-17261317 > . > > > > The reason some nodes may not write snapshots seems to be that when a > new leader is elected, it may opt to send DIFF to the followers if they are > not too far behind. If a follower receives a DIFF, it will not write a > snapshot once NEWLEADER is received. > > > > Is this snapshot write skipped for efficiency reasons, or to maintain > correctness? If it is skipped only for efficiency, I think the upgrade > experience could be improved, by always writing a snapshot at > https://github.com/apache/zookeeper/blob/eeb053767c9e931ae72a2d8c59c0940da3da9679/zookeeper-server/src/main/java/org/apache/zookeeper/server/quorum/Learner.java#L739-L741 > if snapshot.trust.empty=true. > > > > This would allow people upgrading from 3.4.x to set > snapshot.trust.empty=true, upgrade and boot the cluster, and remove the > property again very shortly after the reboot. > > > > What do you think? > Thanks for your proposal. Probably it would be better to create that snapshot not at every "NEWLEADER" but only in the case there is no other snapshot. Do you want to create a JIRA and provide a patch ? Enrico > > > >
