Tag Archives: Reset CBT

VMware Change Block Tracking Issue - Header

vSphere data loss bug returns – CBT issues in vSphere ESXI 8.0 update 2

General, Veeam, VMware, , , , , , , , , ,
The Issue

I keep saying, there are no new ideas in technology, just re-hashes of old ones. That is also true for VMware and their data loss issues.

The vSphere-based change block tracking (CBT) bug is back! I think I wrote 5 articles on this back in 2014/2015 with explanations and fixes!

Veeam reported this at the start of week commencing 11th December 2023, with VMware confirming the issue by the end of the same week.

The Cause

Change block tracking is the feature used to see which blocks of data have changed since a known point in time, to enable backup software to capture only the incremental changes.

If this feature fails, you could lose data in your backups, as the backup software doesn’t know which blocks to protect.

as per VMware:

CBT's QueryChangedDiskAreas may lose some data changed on the disk after disk is hot-extended.
It only happens on ESXi 8.0u2.
The Fix/Workaround

Directly from VMware’s newly published KB, which took them only a few days to confirm this behaviour after Veeam noticed at the start of the week!

  • Resolution
    • Unfortunately, there is no fix available for this bug at this time. However, you can use the following workaround to work around the issue until a fix is released
  • Workaround
    1. Reset CBT after disk is hot-extended. Then, user need to take a full backup immediately.
      It does not fix existing backups, but it makes sure the new ones are good.
    2. Or, user extend disk in offline.

You cannot fix your existing incremental backups if they have been affected, if they missed the correct data to backup, it’s been missed! But you can run an Active Full backup to capture everything, certainly for Veeam this is the case, other backup vendors you’ll need to double check with!

How do I reset Change Block Tracking?

If you are using Veeam, you can just perform an Active Full backup, and ensure the reset CBT option is configured. This is enabled by default.

If you aren’t using Veeam, then the following will be your next steps.

To reset Change Block Tracking, as per this older VMware KB article from the last time this was an issue. VMware may update this article or produce another one now this recent bug has been found.

  • Find your VM in the vCenter Client
    • Power the VM off
    • Click the Options tab, select the Advanced section and then click Configuration Parameters.
  • Disable CBT for the virtual machine by setting the ctkEnabled value to false.
  • If you need to do this for specific virtual disks attached to your virtual machine
    • Disable CBT by configuring the scsix:x.ctkEnabled value for each attached virtual disk to false. (scsix:x is SCSI controller and SCSI device ID of your virtual disk.)
  • Ensure there are no snapshot files (.delta.vmdk) present in the virtual machine’s working directory. For more information, see Determining if there are leftover delta files or snapshots that VMware vSphere or Infrastructure Client cannot detect (1005049).
  • Delete any -CTK.VMDK files within the virtual machine’s working directory.

Now power on your virtual machine.

Depending on your backup software vendor, you may need to manually re-enable Change Block Tracking, you can find a full list of steps and considerations in this VMware KB article. It’s essentially power down the VM, enable in value again in configuration parameters.

Summary

Let’s hope VMware produces a fix for this quickly, I remember they had this issue in vSphere 5.5 and 6.0 and some fixes didn’t resolved the issue, it was a pain being a consultant having to install fixes at customers sites.

It’s good that VMware have only taken a short amount of time to validate this bug and publish something officially about it!

 

Regards

Dean Lewis

Paris Tuileries Garden Facepalm statue

Further ESXi 6.0 CBT bug info – Reset your CBT!!!

VMware, , , , , , , , , ,

Following on from the recent (November 2015) ESXi 6.0 CBT bug, which has now been fixed in the latest released patch ESXi600-201511401-BG, some further information has come to light, provided by Anton Gostev, of Veeam.

You can read the snippet of important information from the Veeam forum post following the issue (Official Veeam KB2075);

All, we have completed the first day of testing in the same exact lab and using the same heavy write I/O test that made the original issue easily reproducible. After a few TB of increments, the above-mention patch appears to fully resolve the original issue when installed on ESX 6.0 Update 1a build 3073146.

However, we found that simply installing the patch is not sufficient, and CBT reset is required for all of your VMs. This is because existing CBT map files may contain issues created earlier due to the original bug, which may result in inconsistent full backups in future. Having CBT reset will also force the following job run use "full scan" incremental pass, thus fixing any existing inconsistencies in backups and replicas, as discussed earlier in this topic.

Provided CBT reset has been performed, Active Full backups is not required.

Performing Active Full backups by itself cannot be considered as a substitute to CBT reset with this particular CBT issue.

Thanks!

You can either follow the CBT Reset instructions from Veeam or look over to Chris Wahl’s latest blog post “Resetting VMware’s Changed Block Tracking (CBT) File with PowerCLI”.

Regards

Dean