Why does the backup use snapshots and not CBT
-
Hi,
i am wondering, why does the XOA backup rely on snapshots and not on Change Block Tracking? I found this article XEN is supporting this feature https://docs.xenserver.com/en-us/xencenter/8-2/cbt-overview.html.
Is this something that's not available in XCP? Are there any plans for this?
cheers Robin
-
Hi,
We did not use it before because it wasn't really useful, since VHD format already provides a way to make a diff. However, using NBD now makes it potentially more useful. Next features of XO backup will rely on it.
-
@olivierlambert thank you for the prompt response.
Do you have any ETA on this feature?
-
I'm not sure to understand the question. Why? What do you think it will bring you?
-
@olivierlambert, fundamentally, it will prevent backup jobs from creating a snapshot for every virtual machine. When you use thick provisioned storage, as we do, it will double the required storage space. With CBT, this is not necessary.
-
No it doesn't. You will need to create a snapshot anyway, but only after you can remove snapshot data to keep the changed blocks only. So sure, you won't keep a snapshot, but you'll need the space to coalesce anyway.
-
@olivierlambert, so it's different from VMware, where a snapshot is created for the backup job and then removed upon completion? In other words, it does not retain the snapshot for an entire day until the next job runs?
-
In CBT, a snapshot is created for the backup and then almost immediately removed. But still, the snapshot needed twice the space of the original disk to be made.
-
@olivierlambert, yes, but the snapshot doesn't need to remain till the next job is running. This distinction significantly impacts space requirements. You don't back up all your VMs simultaneously.
-
Yes, but the space reclaim won't be instant anyway (coalesce must happen first to save space). So the bonus isn't that great. It's better but not that a game changer.
-
@olivierlambert we are further in the process of using xcp, cbt would make a big difference, if u create a backup now on a large and heavy machine u will create a large snapshot during the day, coalesce this snapshot takes hours. If u create a snapshot at ghe start of a backup, copy only changed blocks and then delete the backup u only need to coalesce the small changes created during the backup. For large backups this is a game changer, we currently cannot move large vms to xcp as its backup is not suitable. When will this change? Is this planned?
-
@florent started some experiments with CBT and it didn't go very well. We'll re-discuss that internally.
-
@olivierlambert the current backup method is not suitable for larger vms, if i understand correct on xenserver cbt is allready the standard.
-
CBT is working on XCP-ng too. It's a question of implementation in Xen Orchestra.
-
@olivierlambert ok would be great if we can get this working, will be a huge improvement.
-
@rtjdamen said in Why does the backup use snapshots and not CBT:
@olivierlambert ok would be great if we can get this working, will be a huge improvement.
I concur, I've gotten my backup window down to around an hour, but still the amount of delta in each backup nightly is like 50GB, which doesn't seem feasible given the amount of delta is actually occurring.
-
Are there any 3th party backup solutions on xcp that support this?
-
Reading the doc is a good first step: https://docs.xcp-ng.org/project/ecosystem/
-
@olivierlambert indeed, what about xenserver compatible backups solutions, can they work on xcp as well? I see we could maybe use agent based backups inside the vms for the larger ones, i will test this with one vm, but need a proper solution in the future, i believe if we can get this to work in xoa that would be the best solution.
-
We are using CommVault to backup our XenServer infrastructure. it also does not use CBT but it does not keep the snapshots after the backup. they do some internal tracking. Works very well.
But clearly a CBT solution would be the best way. there is a github repo where you can see how it can be implemented with some simple python scripts.
For now keeping snapshots of all our VMs the whole day to do delta backups is not an option. we are using a iSCSI Storage and dont want to buy that extra SSDs.
Greetings, Franz
