vSphere Changed Block Tracking Support

vSphere's Changed Block Tracking (CBT) identifies the disk blocks that have changed on a virtual machine (VM) since the previous backup, allowing the vmPRO appliance to back up VMs at a differential level. Differential-level backups export only the data that has changed since the last full backup, as opposed to backing up the entire VM disk. When you use CBT with SmartMotion backups, both network I/O and backup times are reduced.

ClosedCBT Cycle
ClosedWhen CBT is enabled for a VM, the vmPRO appliance creates two kinds of files in the /export file system:
  • A file for each base disk (identified by -flat.vmdk), which contains the full disk backup.
  • A CBT file (identified by -pancbt.vmdk) for each base disk. The CBT file contains the changed blocks for the corresponding base disk.
ClosedThe vmPRO appliance performs differential-level backups with CBT as follows:
  1. Using SmartMotion backups, the vmPRO appliance performs a full disk backup following the CBT reset time. The CBT reset time indicates when to initiate a full disk backup, thus resetting the CBT cycle. For more information about CBT reset times, see Configuring CBT on a vmPRO Appliance.
  2. After the full backup, the vmPRO appliance runs differential backups on the disk until the next reset time, at which point the vmPRO appliance runs another full disk backup to replace the previous full disk backup.

Note: Keep in mind that the base disk's modification time is the start of the reset time, while the CBT files have newer modification times.

Figure 1: CBT Cycle

Note: The CBT files are not complete disk images because they contain only the disk's changed data. You cannot access the data in CBT files until you perform a full recovery of these files using the Recover Virtual Machines Wizard.

CBT Considerations

Keep the following in mind when using CBT:

ClosedVersions
  • VMs must be running virtual hardware versions 7 or higher.
  • We recommend using only paid versions of VMware ESX or ESXi servers. Free versions have various application programming interface (API) limitations that keep the vmPRO appliance from functioning as designed.
  • The ESX server must be version 4.0 update 2 or above.

    If you are using ESX 4.1, reverting a snapshot resets CBT for that VM.

    Resolve the reset

    1. Remove all snapshots from the appliance.
    2. On the VMware vSphere Client console, enter the following command:

      cbt reset <hypervisor> <VM>

ClosedVM Transfers

Transferring a VM with Storage vMotion disables CBT.

Standard vMotion (when only the ESX host changes for the VM, but the VM disk stays in the same storage location) does not disable CBT.

ClosedLoad Balancing

Using vMotion on VMs for load balancing during a backup can cause the backup to fail if you do not have one of the following:

  • vmPRO 3.0.1 and newer

  • vCenter Server version 5

In these versions, vMotion is disabled on the VMs prior to the start of the backup process.

For vCenter 4.x, you need to configure the VMs so that vMotion does not run during a backup.

ClosedMultiple vmPRO Appliances

Multiple appliances cannot manage the same VM using CBT.

ClosedActivating and Deactivating CBT

Activating or deactivating CBT on a VM triggers the immediate and simultaneous creation and removal of a VM snapshot. This function is required by the VMware API.

The removal and creation of a snapshot for each VM can be a time-consuming process when activating or deactivating CBT on multiple VMs.

ClosedCBT and Retention Schedules

During differential disk backups, CBT needs to access data from the flat.vmdk file and the corresponding database entry. For this reason, the retention schedule for the backup policy does not initiate partial deletions of full backups. Instead, full backups,differential backups, and associated database entries are deleted at same time by the same retention schedule.

 

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