So I ended up going with the Windows B&R as i could net setup the windows mount server when setting up the repo for the backups. It would setup the linux one but not the windows one. This was with using the veeam console from a windows client. So i ended up with the Windows one. Now after a few backups i am see this warning / error.... 8/5/2026 5:34:50 PM Warning : Failed to use CBT: [Task e75c4247-2ee1-7b51-088c-b970ace60f3d (Async.VDI.list_changed_blocks) failed: . SR_BACKEND_FAILURE_460. . Failed to calculate changed blocks for given VDIs. [opterr=Source and target VDI are unrelated] First i thought it was because i maped the new job to older backups from Beta v1. So i purged all old backups and started fresh. The full backups were successful. Now on first delta 4 out of 7 vms have that warrning. This a Veeam issue or xcp-ng?

This might have worked, at that time i've allready created new VMs from the information inside the Backup xml file. It worked out allright but building VMs this way, is quite tidious. Maybe there could be a way to find the correct installer and patches, to get a new host in a state to restore a pool backup, would be good.

A remote is not the same thing as a SR. For a remote, it's simple: disconnect it (click on "Enabled" so it's disabled then), do whatever you need on the NFS server, re-enable.

I think this is fixed in 6.7: when a delta backup falls back to a full, the task now shows the actual reason instead of just contradicting itself the way you and Andrew both saw (PR #10148). Worth checking after you upgrade.

Both things reported here look fixed in 6.7: the retry succeeding but the job still showing as failed (PR #10129), and the EEXIST: file already exists error, which happened when a job got recreated after a crash (PR #10139). Curious if upgrading clears it for you all too.

The progress bar issue you and Florent were chasing looks fixed in 6.7 (PR #10133). Worth upgrading and seeing if it moves now instead of sitting still. Separately, since this thread is specifically about QCOW2 backups: 6.7 also fixes an offset bug that could leave incremental backups mostly empty after the first block when NBD isn't enabled. Worth a look at the release notes if that's your setup too.

Good news that it sorted itself out. Both changes went in at once and the next run came back clean on the ADDS VMs too, so I don't think there's anything left to pin down here, and I'd rather say that than invent an explanation after the fact. Your question about the snapshot modes was the useful thing to come out of this thread. I went looking, they genuinely aren't documented anywhere, and that's written up on our side now If the fall back to a full ever comes back on the ADDS VMs specifically, that would deserve its own thread with the SMlog output @tjkreidl asked for, since nobody got to look at that part.

Thanks for the feedback @norpan , worth it! Ping @florent

Got this issue since 2026-07-12.

@pierrebrunet said: Hi, just to be sure, are you saying it is working ok apart one task 8 days ago? Yes, after I adjusted the time it's working

@simonp , thank you very much and all of your team ! ! ! Keep up the outstanding work. Regards, Marc

Indeed, it should be detected/reported correctly to avoid losing time finding the problem. Let me ping @julienXOVates

Ping @julienXOVates

Ping @julienXOVates

@Pilow pretty much

@pierrebrunet ticket created https://help.vates.tech/#ticket/zoom/59723

@poddingue Since setting NBD=1, I have not seen the problem. SR is NFS on dual 40G ethernet with a TrueNAS scale 25.10 server using all NVMe SSD, so storage performance is as good as I can make it. I'll have to enable NBD=2 again to see if it still happens and if I can find the relevant part of the logs. As this is a random problem I can't recreate it on a normal test environment.

Pilow's right that moving a VM to another SR forces one full pass while the CBT bitmap is rebuilt; that part is expected. But your screenshot actually shows the likely culprit for the all-VMs-fall-back-to-full pattern: you have Purge snapshot data when using CBT enabled, and XO's incremental backup docs flag exactly that combination as a known issue where you can occasionally get unexpected fulls: https://docs.xen-orchestra.com/xo5/incremental_backups#known-issues. It might be worth running a few jobs with that toggle off to see if the deltas hold. It is a known rough edge on the CBT side, so following the central CBT feedback thread and maybe a nudge to @Team-XO-Backend wouldn't hurt.

@henri9813 said: Hello, I see also this behavior which is "new" since few weeks. Previously, when a backup start: it stake a snapshot ( if there another one before, it delete it ). it upload the snapshot as a backup it coalesce the backup on the remote. end of the game. Now, the old snapshots are not deleted anymore which can lead easily to some disk full. Even with a retention of 1, the problem is present. I observe this only in Backup job, not DR/CR job. I just updated my XO to latest version, i will see if the issue is fixed. Hi @henri9813 , Issue should be resolved in 6.5.x, can you confirm on your side ? Thanks!