" can't compute delta" & "can't connect through NBD, fall back to stream export" after 2026-07-28

[poddingue]

Small follow-up on the NBD connections idea, because I asked internally and it turns out that was not a false lead at all.
The number of NBD connections is capped per host, so 20 per disk is well past what the host will actually hand out, and it is worth turning down whatever else is going on in this thread.
I do not have the exact ceiling yet. I have asked the XAPI side for the real number and I will post it here once I have it, because guessing at it would not help anyone.
@Andrew, that fits your side too as far as I can tell, since you are at 1 and have seen this once rather than constantly.
probain, I will send you an upload link by direct message so you have somewhere to put the logs. I still do not know whether the fallback itself is new this week or simply newly visible, so please do not read anything into my message on that part.

[probain]

@poddingue

Thanks for looking into it further as well.. If there is a hard cap on the amount of NBD connections, then that is very interesting. Because the closest I've ever been able to find regarding this setting, was the general answer of "Try and see what works for you". So if ther is indeed a hard cap, then I would propose that this is limited inside the GUI itself. e.g. if the cap is 8, then don't let the integer exceed 8.

I'm doing increasing increments of the number of connections now. So far I've done 1,2,10. And 10 does seem to be breaking again, but only on some.

Thanks as well for the link. I'm collecting the logs through xen-bugtool --yestoall now. I do wish that something like this is implemented in GUI someday. And I even made a FR feedback on it. Since I really do want to provide good information for the devs. But collecting manually isn't the most convinient way unfortunately.

Logs should be up shortly

[probain]

5 nbd connections went fine.
8 is not.

[olivierlambert]

Happy to read the logs when you can provide them!

[acebmxer]

Just to confirm you are talking about this setting here in the backup job?

If so is this setting supposed to be set to the number of physical nics that can commicate to the backup remote? Or is this more about how many disks at once can be backup over those connections?

Previously had set to 2 or 4. Upped to 6 and see no difference.

[olivierlambert]

Logs are providing interesting results. Discussing internally about it in the next week to see what could be improved about it.

[probain]

@olivierlambert
Happy to hear that they are providing results to be looked at.
If there is anything else I can provide, please don't hesitate to ask. And I'll do my very best to be helpfull

[IgorGlock]

I am not sure if it is the same issue or not.
I doing daily replication from:
4 VMs nvme-1 SR vhd --> nvme-2 SR qcow2
2 VMs nvme-2 SR qcow2 --> nvme-1 SR vhd

That is homelab for testing only.
Xen Orchestra, commit 6a441

Standalone "Host up-to-date!" with latest august updates.

Replication qcow2 --> vhd works as expected.
Replication of vhd --> qcow2 creates daily new replica VM and reports this:

Log: 2026-08-30T18_42_49.945Z - backup NG.json.txt

[IgorGlock]

My reported issue is gone with build 7a187 from sourcecode.

Day before:

tasks	
0	
id	"0mtj9shwq-k2neggnx0sf"
properties	
id	"aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeca"
name	"backup VM"
type	"VM"
name_label	"unifi-server"
progress	0
start	1788303601322
status	"success"
warnings	
0	
message	"can't compute delta OpaqueRef:aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeee76 from OpaqueRef:aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeee32, fall back to a full"
tasks

Day after:

1	
id	"0mtktio1u-lrexahnlve"
properties	
id	"aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeca"
name	"backup VM"
type	"VM"
name_label	"unifi-server"
progress	0
start	1788397201218
status	"success"
tasks	
0	

[probain]

I've since reduced the number of NBD connections to 4. However, I'm actually still seing this happen. I thought it was resolved. But going back to confirm, proved I assumed wrong.

I have therefore uploaded a new bugtools archive from the host server on the link @poddingue sent me earlier.

I hope they can prove helpfull. If you'd like me to do anything else, then I'll happily help out.

Cheers!