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Restarting CBT process


Mauritz Swanepoel

Question

We're working on incremental backups for Citrix Hypervisor using the CBT route. I just want to confirm:

 

1) I enable CBT via command line

2) I then disable it

3) I then re-enable it

 

By disabling and re-enabling it, does it create the CBT process from the start or does it still continue where it left off? As there may be a bunch of empty VDI snapshots that was linked to the original CBT enable, does it restart the CBT process from that point in time or attempt to continue where it was left off?

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On 4/15/2021 at 3:06 PM, Mark Syms said:

When you disable CBT the empty snapshots will be reaped by the background garbage collection process (not instantly as the process can take some time). When you re-enable it will starty again from scratch.

 

Thank you Mark, we've been able to complete this leg of the development, with the last bit we're unsure about. Does the NBD connection to the VDI support writing back? I've been pulling my hair the last week trying to get it working. In the example nbd_client.py there is a write function, however, we're getting an assert error (the response on write returns the following in parse_reply):

 

NBD response magic='67446698' errno='1.000000e+00' handle='0'

 

We have no idea how to debug this. Ultimately we want to take the export we got from the 1 hypervisor and then import that same data back to the failover VDI using the same mechanism from the export. 

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On 4/15/2021 at 3:06 PM, Mark Syms said:

When you disable CBT the empty snapshots will be reaped by the background garbage collection process (not instantly as the process can take some time). When you re-enable it will starty again from scratch.

 

Further to my reply, having read most of the NBD protocol details, it appears that error 1 refers to NBD_EPERM (1), Operation not permitted, leading me to believe that the NBD server does not allow for us to write back, only read. Is this accurate?

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16 hours ago, Mark Syms said:

Yes, the NBD server runs in read-only mode.

 

Thank you Mark, I suspected this was the case, but also cannot understand the logic behind such a decision, as this eliminates the ability to do a NBD > NBD backup restore (which would be the fastest possible solution for small setups wanting some kind of high availability / failover solution). Sure we can now perform incremental backups, but to restore a VM (specifically a large one where we have 1-2TB of storage):

 

1) Rebuild the VDI (as one may have 7 days of "snapshots" from the base) - This in itself will take a long time to complete

2) Then once this has completed, only then can you create and import the VDI data back into the hypervisor 

 

By allowing for NBD connections to write, one could export the entire VDI, import that same VDI into the failover and then only export the incremental data and reimport it using NBD. As CBT only works with full VDI's, there is no risk as both source and target will be the same identical file.

 

The CBT implementation to me now seems to be flawed as the xenapi has previously already had a incremental backup solution in place, which does not require full VDI's but could also work with normal thin provisioned VM's. There is also the ability to rebuild the incremental backup directly onto the base (I will be honest I don't know the background mechanisms of what happens behind the scenes with the --progress operation) and more importantly it does not require Enterprise licenses (which is 2x the license cost of standard licensing). 

 

What benefits are there then left from using the CBT solution VS the xenapi solution presented here? https://xapi-project.github.io/xen-api/snapshots.html - Are there things we're not considering that would leave the CBT solution as a better option than the (free) delta backup solution?

 

 

 

 

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