It’s been a while since MyXWIki.org was not upgraded and since then a few important migrations have been added (various changes of table and column types and size, which can be quite expensive on tables with a lot of entries since it often means copying and deleting all that data in practice).
The migration just started, and I will notify here any progress.
MyXWiki.org is migrated and usable for most people, but we noticed some migration issue with a few wikis and are currently investigating it.
There might still be a few restarts to test some fixes.
Should be ready now. Announcing it.
Thanks 
. Ugrade performed smoothly on my “free XWiki hosted on the community farm”.
I’ve tried to switch indexation form SQL DB to Solr Engine … it looks like it’s yet working again. Can you please confirm? (did performance issues were solved?). If so it’s a very very good news 
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The status tells me Solr is done indexing. An important detail compared to old days (but it was already the case before the upgrade): each wiki has its own independent Solr index sync, which is triggered only when a wiki is accessed.
An important performance issue for this kind of environment (huge number of users) was identified and fixed (which is why I wanted to upgrade to try). I don’t want to say “victory” just yet as there are still other possible sources of performance problems in an instance where so many people have the right to write scripts plus MariaDB is swapping quite a lot. We’ll see how it goes after a while, but it does look reasonably OK so far.
Thanks for this detailed answer 
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I confirm Solr search indexation is finished and is working fine 
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I have also noticed that pages display are far more quicker (even before my personnal Wiki update to 13.9) and that’s also a very good news.
Have a good day and go on for the good job 