On 12/30/2021 9:46 PM, Rahul Goswami wrote:
What is the reason for blocking the upgrade ? If someone has been able to successfully upgrade an index written in 6.x in the past to 8.x without doing a full reindex, I would love to get some ideas.
I'm told that Lucene has never guaranteed that an index can be upgraded through two major versions, even if going to the interim major version first. The guarantee is one major version back, which means 8.x versions can read indexes originally built by 7.0 and later.
The Lucene IndexUpgrader utility just does a ForceMerge on the index. (Solr calls it Optimize, Lucene calls it ForceMerge). The fact that the original segments were written by a 6.x version is preserved by a 7.x ForceMerge, and when 8.x sees that, it refuses to load the index, rather than risk the index not working correctly.
If you want nitty gritty low level details about what kind of problems might occur from using an index originally built by 6.x in 8.x, you're going to need to consult a Lucene expert on a Lucene mailing list. I do not understand Lucene at a low enough level to provide those answers.
It is always best to be able to do a full reindex from the original source data at any time. Almost all Solr installs will require schema changes as business needs shift, and as the administrator gains more knowledge about Solr. Most schema changes require a reindex, and a reindex is strongly recommended with ANY Solr/Lucene upgrade, even from one minor version to the next minor version.
Thanks, Shawn
