Yes, switch to Solr8!

I would add two more cores - but maybe later (It depends a little bit on environment size).

I know you have been part of this discussion but do not forget ...  Performance difference SOAP with and w/o SSL 

We have cores(and RAM) to spare. We can run all the guests on one host if needed, with the 10GB LAN & SAN links becoming the main bottleneck

SSL not an issue.  Stopped at 18.2 just before having to force it.   Their clients and staff forcing migration to M365 and all that fun I am preparing for.

See this Retain TID titled "Improving Indexing Performance By Adding Threads And Cores" at 

https://support.microfocus.com/kb/doc.php?id=7019817 

Disk IOPS on the .../index8/ file system is a big factor.

I think I will be doing that manual pump of index threads.   This system isn't touched that often that 2 cores was enough for day to day usage, and don't really want to give it too much more if I can.  Perhaps with the 4 cores, 8 threads for indexing? 

we were so close to finishing, was at 97% this morning,  now I see both are back to 78%

<sigh>   deeper dive tomorrow as other things beckon

This SAN auto sets what blocks migrate to spinning, otherwise are on SSD.  Will be watching the IOPs

Finally fixed with help from support.  Not sure which exactly fixed things, but the path is worth knowing if others hit his issue.

- increasing cores and threads as discussed above

- Drop Solr logging from debug to 'just' error

- Switched to Solr8, made a backup copy of the Solr7 index, and then removed the Solr7 indexing.

Those made the indexing run so much faster, such that it avoided what ever was tripping up, possibly relating to both Indexers running, or just time.

One last observation also led to an Idea

So after all the tricks I learned in this, the next upgrade/migration I do will likely just go smoothly without needing this. <sigh>

Thanks for the update of what did it for you.   

Do update us on your next upgrade you do to verify if it does go smoothly from the start.