The mysterious Change Block Tracking (CBT) "retry" option

[Gavino]

I rely on Change Block Tracking (CBT) very heavily in both VMware and Proxmox, in order to massively increase the speeds of my backups.

Both the "Use Proxmox VE CBT" and "Use VMware CBT" have the error option of "Switch to proprietary method". That sounds like a good idea, because I don't want the alternative, which is "fail VM processing", as I do want a backup to complete, one way or another.

Both "Switch to proprietary method" options have these three sub-options:

• Immediately
• On the next retry
• On the last retry

Put simply - I have no idea what to choose. If CBT failed, wouldn't I want to change to the alternative behavior immediately?

I could only find the following in the docs:

 

https://www.nakivo.com/blog/vmware-changed-block-tracking-works/

• Immediately: On error action is applied immediately (except for the Reset CBT action).
• On the next retry: On error action is applied on the next retry.
• On the last retry: On error action is applied on the last retry.

-------------------------------

Not good because you've just basically repeated the option name in the "explanation" (e.g. "the next retry option happens on the next retry.  The last retry option is applied on the last retry") yeah I gathered that already!

When is the next retry? At what point in the job process is that? When is the last retry? At what point in the job process is that? How many retries are there? Are we even talking about the same job run. Maybe the "retry" is when the job is next scheduled to run? Documentation sadly doesn't tell me any of this.

I can't think of under what circumstance, if the CBT couldn't be read the first time, how it could then somehow be read on a retry attempt? It's either there and is readable, or it's not there, or it's there and corrupted. The retry options - are they even needed? Please let me know under what circumstances it makes to choose each of the three options, also why "immediately" option is not valid for resetting CBT when it comes to VMware jobs.

Thanks in advance.

Edited February 23, 2025 by Gavino

[The Official Moderator]

On 2/23/2025 at 11:45 AM, Gavino said:

I rely on Change Block Tracking (CBT) very heavily in both VMware and Proxmox, in order to massively increase the speeds of my backups.

Both the "Use Proxmox VE CBT" and "Use VMware CBT" have the error option of "Switch to proprietary method". That sounds like a good idea, because I don't want the alternative, which is "fail VM processing", as I do want a backup to complete, one way or another.

Both "Switch to proprietary method" options have these three sub-options:

• Immediately
• On the next retry
• On the last retry

Put simply - I have no idea what to choose. If CBT failed, wouldn't I want to change to the alternative behavior immediately?

I could only find the following in the docs:

 

https://www.nakivo.com/blog/vmware-changed-block-tracking-works/

• Immediately: On error action is applied immediately (except for the Reset CBT action).
• On the next retry: On error action is applied on the next retry.
• On the last retry: On error action is applied on the last retry.

-------------------------------

Not good because you've just basically repeated the option name in the "explanation" (e.g. "the next retry option happens on the next retry.  The last retry option is applied on the last retry") yeah I gathered that already!

When is the next retry? At what point in the job process is that? When is the last retry? At what point in the job process is that? How many retries are there? Are we even talking about the same job run. Maybe the "retry" is when the job is next scheduled to run? Documentation sadly doesn't tell me any of this.

I can't think of under what circumstance, if the CBT couldn't be read the first time, how it could then somehow be read on a retry attempt? It's either there and is readable, or it's not there, or it's there and corrupted. The retry options - are they even needed? Please let me know under what circumstances it makes to choose each of the three options, also why "immediately" option is not valid for resetting CBT when it comes to VMware jobs.

Thanks in advance.

Hi @Gavino

By default, each run includes three job retries, with an internal retry interval of 15 minutes. This configuration can be modified in System Settings:
(See the screenshot)

Regarding VMware, there is another option called "Reset CBT", which can only be applied on the next retry. This feature cannot be applied immediately for the current run.

You can refer to this article for more information: 
https://helpcenter.nakivo.com/Knowledge-Base/Content/Change-Tracking-Issues/ESXi-Host-Did-Not-Provide-Valid-CBT-Data.htm

Thank you for choosing NAKIVO. We look forward to hearing from you.