CBT: the thread to centralize your feedback

flakpyro

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.

Rhodderz

@flakpyro ah I will try that once proxy for that pool is back
We upgrade XOA from stable channel to latest as we had another issue which is apparently resolved in that with NBD (causing some machines to go RO)
Once thats fixed I will try again to see if the above update and/or disabling "Purge snapshot" works as a workaround.

We have purge enabled (and would like it left enabled) as we use iSCSI (Dell SC5020's) so everything is a little fat, especially with some clients.
I shal update tommorow on what happens.

flakpyro

@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!

Rhodderz

To add an update and to not leave on a cliff hanger.
We have since updated our XOA to the latest channel to attempt to fix an NBD issue.
This move broke a proxy of ours, but also all the backups are going through the XOA and after this the backups have not had an issue since.
So either the new NBD fixes, it being only on an XOA or something somehwere else resolved this problem for now.

We will be enabling the same in our other pool soon so will update if we have the same issues there.

flakpyro

Sadly the latest XOA release from today does not resolve my strange CBT issue,

[08:32 xcpng-test-01 45e457aa-16f8-41e0-d03d-8201e69638be]#  cbt-util get -c -n 4d7f0341-bbce-4957-a4c4-d603725a807a.cbtlog 
1950d6a3-c6a9-4b0c-b79f-068dd44479cc
After Migration from Host 01 to Host 02 (Shared NFS SR):
[08:33 xcpng-test-01 45e457aa-16f8-41e0-d03d-8201e69638be]#  cbt-util get -c -n 4d7f0341-bbce-4957-a4c4-d603725a807a.cbtlog 
00000000-0000-0000-0000-000000000000

olivierlambert

I don't think that's an XO issue, but more something weird in your XCP-ng setup that nobody can reproduce (but it doesn't mean we couldn't solve it)

flakpyro

@olivierlambert Hmm im really not sure whats unique about my two pools. One is AMD + TrueNAS the other Intel + Pure Storage. If this is actually unique to me only perhaps i would be better off submitting a ticket to help get to the bottom of this?

olivierlambert

You manage to find a CBT issue without using any XO command, which is great because we know it's not XO now I think @dthenot is already taking a look internally.

dthenot

@olivierlambert I am

flakpyro

@dthenot @olivierlambert thanks guys ill hold off on submitting a ticket for now to keep the conversation centralized here but if you need any more info, would like me to try anything or would like a remote support tunnel opened just let me know!

Tristis Oris

can't run live migration to another pool because VDI_CBT_ENABLED. is it intended?

Tristis Oris

@Tristis-Oris even halted VMs can't migrate with snapshot. need to remove it.

olivierlambert

That's weird, ping @florent

Forza

@Tristis-Oris We've had the same problem, so are not using CBT for now.

rtjdamen

@Tristis-Oris migration of a vdi between sr is not supported with cbt enabled. U need to disable cbt first. This is done by xoa. Live migration of vm between hosts is supported as long as the sr stays the same. This is by design on xen

Tristis Oris

@rtjdamen but i can't disable CBT globaly? it auto applied to every VDI when been implemented.
Disable CBT for each VDI not required, because it happens automaticaly during migration. I only need to remove all snapshots.

rtjdamen

@Tristis-Oris indeed seems like thats a bug in xoa that it does not delete the snapshots

jon02

@flakpyro
I have the same problem.
I'm on 8.2 as well and have a local ZFS SR.
I'm going to upgrade to 8.3 and look, if it helps.

flakpyro

Another interesting development. In our test environment this week i installed the latest HP Service pack for proliant, doing so required a server reboot so I ran a rolling pool reboot from XOA, later when the test environment backup job kicked off, i noticed it was running a regular Delta despite the migrations that must have occurred during rolling pool reboot.

SSHing onto a host and checking i see sure enough the cbtlog is reporting all zeros...

[17:27 xcpng-test-01 45e457aa-16f8-41e0-d03d-8201e69638be]#  cbt-util get -c -n 73877c18-a5bf-43bb-aaf5-299f46710d7e.cbtlog 
00000000-0000-0000-0000-000000000000

However the backup ran as a delta, running the backup again manually and it is once again it runs as a delta.

Checking after the manual backup the result is not all zeros anymore:

[17:28 xcpng-test-01 45e457aa-16f8-41e0-d03d-8201e69638be]#  cbt-util get -c -n 65d8656e-93e8-4e81-b1a8-0b0462f6fbb8.cbtlog 
1950d6a3-c6a9-4b0c-b79f-068dd44479cc

Now..just for fun i decided to manually migrate a small VM to another host and then back to see what happens:

After the migration back to all zeros:

[17:32 xcpng-test-01 45e457aa-16f8-41e0-d03d-8201e69638be]#  cbt-util get -c -n 65d8656e-93e8-4e81-b1a8-0b0462f6fbb8.cbtlog 
00000000-0000-0000-0000-000000000000

And running a backup manually resulted in the usual error:

Can't do delta with this vdi, transfer will be a full
Can't do delta, will try to get a full stream

So...this just makes the issue even more confusing, why does a rolling pool reboot not cause this behaviour but a manual migration does? Does the ID being all zeros not actually matter? I seem to be able to consistently reproduce this too. Ill be curious to next test if a "rolling pool update" causes this behaviour next time a batch of updates is released.

rtjdamen

@flakpyro very strange issue indeed, we can’t reproduce the problem on our end. So as @olivierlambert mentioned before it has to be something specific. The new developments make it indeed more difficult to understand, but i believe there is a logical explanation to this.