CBT: the thread to centralize your feedback
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@jimmymiller as part of the migration the cbt should be disabled allready, this feature has been created as part of the first release, however it seems that there is a bug that does only disable cbt but leaves the metadata only snapshots on the vm, i believe this is causing the issue.
I think this pull request is created to solve this in the next release
https://github.com/vatesfr/xen-orchestra/pull/7903 -
So testing CBT in our test environment with migrations and this is what i have observed:
Host 1 and Host 2 are in a pool together with a shared NFS SR. If TestVM-01 i son Host 1 using the NFS SR with CBT backups enabled all is fine. Clicking on the VM and then disks shows that CBT is enabled on the drives. If i migrate the VM over to host 2, CBT is disabled and the VM is migrated successfully. On the next backup job run however the job will initially fail with the errror "can't create a stream from a metadata VDI, fall back to a base "...after a retry then the job will run.
If multiple jobs exist for a VM. Say a backup job and a replication job, will that result in 2 CBT snapshots then? That is a ton of space savings vs keeping 2 regular snapshots with the old backup method and cuts down on GC time and storage IO by quite a bit!
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@flakpyro that’s exactly the reason we were asking for the cbt option to come available. It’s a huge difference in storage usage and the amount of writes done to the storage. Huge improvement!
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In theory, migrating a VM to another host (but keeping the same shared SR) shouldn't re-trigger a full. It only happens when the VDI is migrated to another SR (the VDI UUID will change and the metadata will be lost).
At least, we can try to reproduce this internally (I couldn't on my prod).
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@olivierlambert Looks like since the last XOA update it no longer triggers a full which is great news. Instead after a migration i see "can't create a stream from a metadata VDI, fall back to a base" when the job next runs. After that it retries and runs as usual.
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Interesting
Where are you seeing this message exactly? -
@olivierlambert
For me it appears in the backup summary on the failed task
And after the job retries. (I have retries set to 3)
I believe others a few posts up are also seeing this same error message. @rtjdamen @manilx
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And only after migrating the VM from a host to another, without changing its SR?
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@olivierlambert Correct. The VM in question is on a NFS 4 SR. The next backup run will run without the error for as long as it stays on that host, however if i move it again the process repeats it self.
We are running paid XOA so if it helps i can enable a support tunnel for you guys to take a look at logs. However we have quite a few nightly backups now so not sure if the logs will have rotated. Either way its in our test environment so we can run the job anytime to generate fresh data.
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That's weird, I'm using RPU in our prod so the VM are moving, but the VDI doesn't change. Anyway, another test to try internally
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@olivierlambert we do not see this error in relation to migration, they just happen at a random situation. I have seen issues with cbt and multidisk vms, could it be that these are multi disk vms?
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@rtjdamen Yes the VM i have been testing with above is a multi disk VM. I will do some testing with a single disk VM and see if that makes a difference.
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@flakpyro this issues we saw were only happening on nfs and multidisk vms. The error was cbt was invalid if i am correct. This is not a xoa error but we saw it on my 3th party backup tool also using cbt
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@rtjdamen After testing with both single disk VMs and multi disk i am seeing it on both. However it is much more common on multi disk VMs.
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@flakpyro are u able to test it on iscsi?
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@rtjdamen I dont not have any iscsi luns mapped to our hosts to test with. We went all in with NFS from iscsi as we moved from ESXi to XCP-NG.
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After the CBT feature was added to the backup method, The error "VDI must be free or attached to exactly one VM" was occurring on our system. Today I upgraded XO to the e15f2a1 and tried taking full and delta backups with Continuous Replication. Now everything works fine.
NBD connection enabled, NBD+CBT backup enabled, XCP-ng 8.2, CR backup from FC storage to NFS storage. -
Great news! Other similar feedback in here?
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@olivierlambert can’t check, running on xoa.
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It will be available in 12 days then
