@stormi Updated a test machine running only couple VMs. Everything installed fine and rebooted without issue.
Machine is:
Intel Xeon E-2336
SuperMicro board.
One VM happens to be windows based with an Nvidia GPU passed though to it running Blue Iris using the MSR fixed found elsewhere on these forums, fix continues to work with this version of Xen. 
@CodeMercenary I am using V4 on the XO-Server to our backup remotes and it seems to work just fine. However using V4 as a storage SR was nothing but problems, as @nikade mentioned we had tons of NFS Server not responding issues which would lock up hosts and VMs causing downtime. Since moving to v3 that hasn't happened.
Checking a host's NFS retransmissions stats after 9 days of uptime i see we have had some retransmissions but they have not caused any downtime or even any timeout messages to appear in dmesg on the host.
[xcpng-prd-02 ~]# nfsstat -rc
Client rpc stats:
calls retrans authrefrsh
268513028 169 268537542
From what a gather from this blog post from redhat (https://www.redhat.com/sysadmin/using-nfsstat-nfsiostat) it seems like that amount of retransmissions is VERY low and not an issue.
@gduperrey Installed on a 2 host AMD based test pool, as well as our 5 host Intel based production pool without issue using rolling pool update. Everything migrated, updated then migrated back perfectly.
Also installed on my home server without issue.
@bleader Updated a small test server without any issues.
@olivierlambert Im making progress getting to the bottom of this thanks to some documentation from XenServer about using cbt-util.
You can use the cbt-util utility, which helps establish chain relationship. If the VDI snapshots are not linked by changed block metadata, you get errors like “SR_BACKEND_FAILURE_460”, “Failed to calculate changed blocks for given VDIs”, and “Source and target VDI are unrelated”.
Example usage of cbt-util:
cbt-util get –c –n <name of cbt log file>
The -c option prints the child log file UUID.
I cleared all CBT snapshots from my test VMs and run a full backup on each VM. Then ensured the CBT chain was consistent using cbt-util, the output was:
[14:22 xcpng-test-01 45e457aa-16f8-41e0-d03d-8201e69638be]# cbt-util get -c -n 867063fc-4d86-420a-9ad2-dfe1749ecbc1.cbtlog
1950d6a3-c6a9-4b0c-b79f-068dd44479cc
After the backup was complete i then migrated the VM to the second host in the pool and ran the same command from both hosts:
[14:26 xcpng-test-01 45e457aa-16f8-41e0-d03d-8201e69638be]# cbt-util get -c -n 867063fc-4d86-420a-9ad2-dfe1749ecbc1.cbtlog
00000000-0000-0000-0000-000000000000
And from the second host:
[14:26 xcpng-test-02 45e457aa-16f8-41e0-d03d-8201e69638be]# cbt-util get -c -n 867063fc-4d86-420a-9ad2-dfe1749ecbc1.cbtlog
00000000-0000-0000-0000-000000000000
That clearly is the problem right there, question is, what is causing that to happen?
After running another full the zero'd out cbtlog file is removed and a new one is created which will work fine until the VM is migrated again:
[14:39 xcpng-test-01 45e457aa-16f8-41e0-d03d-8201e69638be]# cbt-util get -c -n 1eefb7bf-9dc3-4830-8352-441a77412576.cbtlog
1950d6a3-c6a9-4b0c-b79f-068dd44479cc
We use ZFS on our backup target, mounted to XOA via NFS. Works without issue.
@rtjdamen Makes sense, it would be nice is XOA cleaned up these small CBT snapshots though after a migration from one SR to another.
Same issue here, i posted on discord about it but perhaps the forum is a better place for it.
I see the the newer releases of XO from source adds CBT to backups and enables it by default. Anyone else having issues with this feature? All my VMs were stuck "exporting content" in my test environment this morning, also looping tasks consisting of "call: sr.getAllUnhealthyVdiChainsLength" and under the health dashboard all my VDIs were unhealthy needing to coalesce.
What fixed it for me was to roll back to a pre CBT release, and remove all the .cbtlog files from my SRs, i had to delete all my VM snapshots, let garbage collection run, then ran a fresh backup run.
@florent Thats great news! We use both Borg and Veeam so that's where most of these ideas are coming from. Having GFS backups like you mention for weekly, monthly, yearly would also be awesome to have and something we make use of in Veeam currently.
@mbven Strange we have a few Windows Server 2016 servers remaining that seem to be working since since the last batch of updates. I even tried to reboot one just to be sure. They are running "Microsoft Windows Server 2016 Datacenter". What CPU are your servers running? Any errors in xl dmesg when the crash occurs?
@Rhodderz I agree we are using NFS so snapshots are thin at least but we would love to be able to delete the snapshots after a backup run as well. Hopefully in time we can get this working!
For our production pool i have CBT + NBD enabled but i have "Purge snapshot data when using CBT" disabled. The results in successful backups but the snapshot is retained. I assume it then ends up using that snapshot for the following delta backups.
@florent Testing a storage migration i do see CBT get disabled and reset during the process which is expected! I do notice it leaves the .cbtlog file on the old SR after the storage migration is complete but that's easy enough to clean up manually.
The issue i posted above however is just a VM migration from host to host on a shared NFS SR, the SR the VM is on is not changing.
@florent thanks for checking into this as we'd love to be able to use this feature. If you need me to test anything or provide any additional logs/info about our environment let me know!
@olivierlambert So i guess the next thing we need to do is have someone also running 8.3 test this using an NFS SR?
@rtjdamen Both pools are on 8.3 with all the latest updates.
I did find this PR on github and wonder if it may be related: https://github.com/vatesfr/xen-orchestra/pull/8127 but not sure why it would only happen after a migration....
Looks like it does this if the VM is powered off as well. Im really not sure what else to try since this is happening on 2 different pools for us.
I may need to end up submitting a ticket with Vates for them to get to the bottom of it.
This is a completely different 5 host pool backed by a Pure storage array with SRs mounted via NFSv3, migrating a VM between hosts results in the same issue.
Before migration:
[01:41 xcpng-prd-03 b04d9910-8671-750f-050e-8b55c64fbede]# cbt-util get -c -n 83035854-b5a9-4f7e-869f-abe43ddc658d.cbtlog
e28065ff-342f-4eae-a910-b91842dd39ca
After migration
[01:41 xcpng-prd-03 b04d9910-8671-750f-050e-8b55c64fbede]# cbt-util get -c -n 83035854-b5a9-4f7e-869f-abe43ddc658d.cbtlog
00000000-0000-0000-0000-000000000000
I dont think i have anything "custom" running that would be causing this so no idea why this is happening but its happening on multiple pools for us.
@rtjdamen Hmm very strange.
The only thing i can think of is that this maybe due to the fact these VMs were imported from VMware.
Next week i can try creating a brand new NFSv3 SR (Since NFS4 has created issues in the past) as well as a new clean install VM that was not imported from VMware and see if the issue persists.
@rtjdamen interesting, this is with iSCSI (block) or with an NFS SR?