@olivierlambert Yes I was able to do that. Just not the other direction.

I've just been through this loop. You can't even migrate halted VMs cross-pool from 8.3 to 8.2.1, although it's possible the other way around.

I decided to install 8.2.1 on the new server (which isn't scary if its a new server, not doing anything ...)
I then joined it to the pool and applied all outstanding 8.2.1 patches on all 3 servers, using the CLI because XO wasn't showing the patch status consistently: yum repolist (to make sure only xcp-ng-base and xcp-ng-updates are enabled); yum check-updates; yum updates on each system, pool master first.

Then I made the new server pool master thinking it would be safer to upgrade the new, empty host first. For 8.3 the upgrade is done from the iso media (on a USB stick). The upgrade was smooth.

I then ran into a glitch: I wasn't able to do certain operations on the pool because it was in mid-upgrade. I did live-migrate some vm's from old to new, but I shut one down hoping to reduce memory size so it would fit on the new server and it wouldn't let me reduce memory on the vm - "mid upgrade". And I wasn't able to start VMs on the old 8.2.1 pool members - "mid upgrade". Ended up shutting down all but a few essential vm's that had been live-migrated to the new server - leaving the two old servers empty.

Next glitch: my USB stick is UEFI, the old hosts were Legacy BIOS. Changed the servers to UEFI and was able to boot the USB. However the installer said the existing system on disk couldn't be upgraded - perhaps because of the BIOS/UEFI issue, not sure.

I ended up reducing the pool to the new server only and doing a clean install of 8.3 on both of the old servers. I lost all of the vm virtual disks on old servers "local storage", not a disaster, almost everything was on the shared storage pool but there were some VM's that could have been useful. And then of course I joined the two old servers to the pool again.
Then there were 72 v.8.3 patches to apply to the 3 systems, from XO.

I should have moved all the VMs around before starting the upgrade. Because I didn't do that, it took a lot more time than I expected and was nail-biting at times, but it ended well enough.

@stormi Doh. How did I miss that?

Hi, I've just applied the May and two June security updates (total of 8 patches) to a test box, no problems. But before I try these on the live pool: can anyone tell me if a reboot is required? The blog posts referenced do not mention this.

@olivierlambert Merci!

Hi, I'm confused by different versions of documentation for XO, Xenserver etc. and concerned about the recoverability of XO backups of SQL and Exchange server VM's.

1. Does an XO backup with advanced option "normal snapshot" use VSS to provide application consistency? I.e. does it use vm-snapshot-with-quiesce or a normal vm-snapshot?
2. Some sources say the above only works with an additional installation step but it is unclear what that is, the one I found seems to be obsolete per: https://xen-orchestra.com/blog/xenserver-quiesce-snapshots/ If I do vssadmin List Providers the Citrix VSS does NOT appear.
3. https://xen-orchestra.com/docs/backups.html#consistent-backup says that quiesced backups (i.e. quiesced snapshots) are deprecated in favor of RAM-enabled backups. Much slower and larger. Is that the only or the best way to ensure apps can be restored without data corruption?
4. What IS the recommended snapshot method, to ensure backups taken by XO are 100% safe, notably in the case of complex Windows apps like SQL and Exchange?

@julien-f Excellent, thank you!

@normanghenderson Hi Julien, Happy New Year. Just touching base on this? I just made an attempt with xo-server 5.73.0 xo-web 5.76.0 and got the usual SR_OPERATION_NOT_SUPPORTED due to the large disk.

@julien-f Thanks very much. Let me know when there is something to test.

Here's another case in XCP-ng/Xen, which pre/post hooks for XO backups could be used to solve:

Any way to backup VM with USB attached?

There are many situations in which it would be desirable to shut down a VM before backup and start it up again afterwards, especially these cases in which a storage device or USB device needs to be detached or the backup won't run...