New 6.4 installations with ZFS will no longer have any issues with zpool upgrades and issues with GRUB on reboots - by default?
Exactly. In new 6.4 based installation Proxmox VE makes GRUB boot from the quite easy to handle ESP vfat partition to avoid any issue booting from ZFS directly, that is then done by the initial Kernel + initramfs disk which uses the same upstream OpenZFS code base the actual system will use.
One may still want to wait upgrading the rpool, as and sometimes one needs to boot into an older kernel (if the newer one has some independent regressions) and said older kernel may not know about all features yet. But that is nothing new and something you can run into with any upgrade including new features in a filesystem that older versions cannot understand, and in general way better than depending on the external ZFS support in GRUB.
So, a general applicable process to make a bigger upgrade safer to do could look like:
- Upgrade to new ZFS + Kernel
- See if the new kernel works OK for a week or so, most regressions show up much quicker, so the extended period is just to be safe
- Only then do the ZFS pool upgrade.
2. I'm assuming the howto on switching to this new boot mode will indicate whether this be seamless on 6.3 systems that are being upgraded and that are RUNNING, and will not require any reboots?
Yes, it's meant for a live and running system and should not interrupt any guest/storage workload.
A single reboot is required to activate the new way and to ensure the switch worked, but it is not necessary to do that immediately.