Interesting! I will try to upgrade some customers tomorrow. First some smaller ones (appr. 1 TB), then larger ones.

Hopefully I have feedback soon.

I started upgrade on very small system with 40 GB. The indexer upgrade starts right after the upgrade and it uses plenty of CPU. Resulting index files are twice as big as the old ones. 

I upgraded from 4.10.1.1 to 4.11.0.0 this week and all went well.  I let the Solr 8 do its indexing for a day or so, then I switched over to Solr 8 then deleted Solr 7.  Now IP Port 8082 is used for my Solr 8.  The only weird thing I noticed so far is using the "Advanced Search" does not find all the items like it did in 4.10.  I'm trying to debug that now.

Mine is still preparing a new Solr8 index (and killing my ESX server with high load).

So for larger environments I have to take care of these small details 

I found, that in Server Configuration > Maintenance are greyed out "Enable Index Backup" and "Enable Index Optimization" bug or WAD ?

The Retain engineering team is aware of the issue.  Please follow this document as it will be updated as soon as the fix is available.

portal.microfocus.com/.../KM000011785

Same here.  Why did they disable those settings?

So this new index causes some strange behavior it seems.

My environment shows it all.

At one of my customers I miss the possibility to start Solr8 or to switch from Solr7 to 8 or even refresh index information (case has been opened)

They are disabled until the index has been switched to Solr8 and you are confident the index is functioning as it should be.  You then have the option to 'Clean up Solr7' and once that takes place, the options should be enabled.  Here is a link to the documentation that provides more details regarding the update to the new Solr8 indexing engine.  https://www.novell.com/documentation/retain-4.11/install-upgrade/data/upgrade-492-4101_411.html#t4jzht77vz1d  I have submitted a request to the documentation team to add a note in this area of the documentation that states: "Note:  Until the 'Clean Up Solr 7' action as been performed, the "Enable Index Backup" and "Enable Index Optimization" options under Maintenance will be grayed out."

Article https://portal.microfocus.com/s/article/KM000011822?language=en_US has been published.