CBT: the thread to centralize your feedback

flakpyro

@olivierlambert Glad we're getting to the bottom of this!

Out of curiosity is having an isolated migration network only available to the XCP-NG hosts considered best practice with XCP-NG? It was with VMware to keep VMotion traffic on its own subnet and since the VLAN was already created on our switches i decided to keep with that setup. Ideally we can get this fixed either way, I'm just curious if I'm doing something considered strange?

olivierlambert

No it's not strange at all. It's not a bad practice neither

Now, I'd like to check something with xe: migrate the VM but with the migration network selected. To see if this resets CBT or not.

flakpyro

@olivierlambert
Im on it! However after searching the XCP-NG docs as well as the XenServer docs i can't see to find how to specify a migration network using xe from the cli. Are you able to provide me the flag i need to use?

olivierlambert

I don't remember the command but @MathieuRA should be able to tell you which call we do to the XAPI when we add a migration network.

flakpyro

@olivierlambert @MathieuRA once you are able to provide me xe migrate flag to specify a migration network i will test this ASAP. I think we're really close to getting to the bottom of this issue!

rtjdamen

Hi All,

First of all best wished to you all for 2025! I have just deployed the latest build to do some testing on the one remaining issue we have with cbt backups, we were still facing full backups on some vms, this is expected to happen because cbt is not activated fast enough on some vdi’s, i will update this post once it completed some test runs to let u know if this build resolves it (there is a fix inside this build that should fix it).

Robin

olivierlambert

Happy new year and thank you very much for the feedback provided in here

rtjdamen

@olivierlambert my pleasure, good to be a part of it.

Good news, this bug seems to be resolved!

Hope we can fix the migration bug as well!

flakpyro

I think we have a pretty good idea of the cause now, It seems to be related to having a migration network specific at the pool level.

I think we are closer than ever to having this worked out and should help a lot of us using a dedicated migration network. (As was best practice in Vmware land) What are the next steps we need to take?

olivierlambert

We need to wait for the largest part of the team to get back from vacation on Monday

MathieuRA

Hi @flakpyro
You can do xe help vm-migrate to see all available parameters and a small description.

BTW, in XO, if a network is specified for the migration, we call vm.migrate_send otherwise vm.pool_migrate.
vm.migrate_send also migrates VM's VIFs and VDIs.

Questions for the XCP team:

What happens if you do a vm.migrate_send but the destination SR for the VDIs is the same?
Is there a way to call vm.pool_migrate using a specific network?

flakpyro

@MathieuRA

Thanks for the tip!

Looking at the output:

command name            : vm-migrate
        reqd params     : 
        optional params : live, host, host-uuid, remote-master, remote-username, remote-password, remote-network, force, copy, compress, vif:, vdi:, <vm-selectors>

Ir does not appear there is a way for me to specify a migration network using the vm-migrate command?

It sounds to me like vm.migrate_send is causing CBT to be reset while vm.pool_migrate is leaving it intact? The difference between a migration that is known to be kept within a pool vs one that could potentially be migrating a VM anywhere?

olivierlambert

Adding @psafont in the loop because it seems we should use pool_migrate with a network migration and/or migrate_send not resetting the CBT.

psafont

@olivierlambert xe vm-migrate uses migrate_send when storage or network needs to be changed, otherwise vm.pool_migrate is used. Selecting a new network is done through a the vif parameter. This parameter is a map in the form of vif:<VIF_UUID>=<NEW_NETWORK_UUID> vif:<VIF_UUID2>=<NEW_NETWORK_UUID2> (and so on).

So I'm not so sure that a netwrok migration can happen when using pool_migrate.

flakpyro

@psafont

So in the case where CBT is being reset the network of the VM is not actually being changed during migration. The VM is moving from Host A to Host B within the same pool, using NFS shared storage which is also not changing. However when "Default Migration Network" in the pools advanced tab is set on the pool, CBT data is reset. When a default migration network is not set, the CBT data remains in tact.

I seems like migrate_send will always reset CBT data during a migration then even if its within the same pool on shared storage and that this is used when a default migration network is specified in XO's Pool - Advanced tab. While vm.pool_migrate will not reset CBT but is only used when a default migration network is NOT set in XO's Pool - Advanced tab. Not sure how we work around that short of not using a dedicated migration network?

olivierlambert

So either we need to update pool_migrate to allow a migration network parameter, OR we need to be sure than migrate_send do NOT reset the CBT data if the storage do not change. @psafont Do you have a preference on which one we need to do upstream?