Are there any downsides to using CBT when doing backups?
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I moved over to using CBT this past weekend. It seems much, much, faster and I didnt have a single error. In the past about 20% of my nightly incremental backups had errors like "can't compute delta OpaqueRef:blah-fba0-ced3-7653-blahblah from OpaqueRef:blah-b53d-3935-1af0-blahblah, fall back to a full"
I have this option "Purge snapshot data when using CBT" set to yes.
I have both of these options set to no as I don't really fully understand the implications (other than the info bubble
"Merge backups synchronously "
"Synchronize snapshots"With that said, are there any downsides to using CBT when doing backups?
Thanks!
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One potential downside is that CBT makes live-migrations between SRs alot more difficult.
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P poddingue marked this topic as a question
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@jr-m4
What would you need to do if that were needed?? -
Last time I tested CBT enabled backups the backup job would sometimes create a full backup for a given VM instead of an incremental one.
This bug got triggered by live migrations. When a VM got live migrated it got processed as full backup by backup jobs even after "normal" live migrations (no storage live migration). -
If you do the storage migration from XO, you shouldn't have to do anything.
XAPI does refuse to move a disk while CBT is on, as @jr-m4 said, but reading XO's code it catches that refusal, turns CBT off on the VM's disks and retries.
The price to pay is that the next backup of that VM is a full one, and the backup job switches CBT back on.
The other trade-offs are in the docs (https://docs.xen-orchestra.com/backups-and-dr/backup-types/incremental_backups#purge-snapshot-data-cbt) : NBD has to be enabled, snapshot retention has to be 0, and since the purged snapshot only keeps metadata you can't roll back to it.
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@poddingue
Acctually my experience is that a VDI with CBT enabled can't have its VDI migrated at all. And XO does indeed catch that CBT is enabled. But it doesn't disable it -> migrate -> re-enable.@archw
And if you'd like to migrate VDIs between SRs you'd most likely also have to delete existing snapshots as well (along with disabling CBT prior to migrating). -
@jr-m4 : I tried it on a lab pool today. XO does catch the refusal, turn CBT off and retry, but only on the disks of the VM itself. If the VM has a normal snapshot, the snapshot's disk still has CBT on, XCP-ng checks that disk too and refuses the move, and it won't let you turn CBT off on a snapshot. So with snapshots around you have to delete them first, like you said. I had that part wrong.
@archw your setup is the other case. With "Purge snapshot data when using CBT" on and retention at 0, the backup job deletes the snapshot and keeps only a small metadata file that isn't attached to anything. In the same test that file didn't get in the way: once CBT was off on the live disk, the move went through. So for VMs backed up that way, migrating from XO should still work, with a full backup on the next run. A snapshot you take by hand would block it the same way.
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@poddingue
Thank you! -
@jr-m4
Thank you! -
@poddingue
Thank you!
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