OS 6.6 on Ultra 6:. Not ready for prime time? Do I need more memory? Or...?

Thanks very much to all who replied! It's good to know that OS 6 can run well on Ultra 6s, even if that hasn't been my experience.

• Logs were sent to mdgm, but there weren't any apparent smoking guns as I never heard anything back.
• I kept getting hangups where various services stopped responding, typically within a few hours of a reboot.
• Several times, the "reboot -f" required to restart the system resulted in the message "Remove inactive volumes to use the disk. Disk #1, 2, 3, 4, 5, 6." and all the drives on the Volumes page were shown in red. Rebooting again from the web UI restored normal ops, but the overall experience was not encouraging.
• Finally, a hangup occurred such that even ssh access was lost. A back-panel power-switch reboot was done, resulting in the "Remove inactive volumes" state; a subsequent reboot resulted in two inactive volumes: data, and data-0, which further rebooting didn't resolve. Oh, dear!

I could have continued to collect data in an attempt to troubleshoot further, but I had real work to do and NAS-nannying was taking up too much of my time, so four days ago I rolled back to OS 4.2, restored my saved configuration, and restarted my backups. The required resyncs and volume expansions ran under six hour each (not the 180-some hours that OS 6 predicted, but never had a chance to run to completion); the full backup of 8.8 TB of data ran in about two days, and the box is rock-solid stable again.

Might more memory have made the difference? The existing 1 GB memory tested fine, and that amount has worked well in both my Ultra 6s for years. I re-checked memory utilization on both boxes running OS 4 when running large backup jobs and both use about as much swap under OS 4 as the test box did under OS 6, so it's unlikely to be memory exhaustion and swapping per se that's the issue, unless OS6 has introduced new bugs in that regard.

Why my test box failed to behave well on OS 6 while others work fine remains a mystery. But I need my NASes to Just Work, and on OS 4 they do Just Work. Yes, I'll have to do a Factory Reset when I hit certain expansion limits, but I'll live with that inconvenience in return for the rock-solid reliability I've come to expect from OS 4 (not only on my two Ultra 6s, but on the two Pro 6s I set up at my former place of work). "If it ain't broke, don't fix it" turned out to be the right answer in my case.

The good news is that the rollback from OS 6 to OS 4 went flawlessly and my backup NAS is restored to its role as a solid, stable server. It's pretty cool that this box could be updated to an unsupported OS, and even cooler that it could be rolled back, none the worse for wear: mad props are due to mdgm, AMRivlin, and others involved in making these upgrades/rollbacks possible.

Thanks again, all!