@florent Running delta backup with CBT to S3. NBD connections=1 Yes, NBD. No VHD only. Yes, NBD is enabled and working on all pool hosts. Yes, XO can access the network used for NBD. No, a default backup network as not been assigned. No more logs from journalctl. My normal large nightly delta backup to S3 ran and did not produce any new errors. So far, just the one error once.

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

@pedro_udifar Hi, You can try again with minPartSize=100000000 indeed. But I would advise you to contact B2 directly. They should have a clearer error to show instead of a 500. Thanks for your patience and your help

Got this issue since 2026-07-12.

@Bastien-Nollet Perfect, thanks Bastien! Thanks for looking into this, I really appreciate how responsive everyone at Vates is on this forum.

@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

@christopher-petzel Thanks!

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.