Changed Block Tracking

The Citrix Hypervisor changed block tracking feature offers incremental backup capabilities for customers using Citrix Hypervisor. This feature is available only for Citrix Hypervisor Premium Edition.

When changed block tracking is enabled for the virtual disk images (VDIs) of a VM, any blocks that are changed in a VDI are recorded in a log file. Every time the VDI is snapshotted, this log file can be used to identify the blocks that have changed since the VDI was last snapshotted. This provides the capability to backup only those blocks that have changed.

Changed block tracking can be enabled by using the CLI or API. It is usually enabled by the third-party product that you use to take incremental backups. When changed block tracking is enabled for a VDI, additional information is calculated and stored that lists the changed blocks for the VDI. This process uses resources such as memory and space.

For more information about changed block tracking, see the developer documentation.

Disabling changed block tracking on a VM

You can disable changed block tracking for all VDIs associated with a VM by using XenCenter. Before disabling changed block tracking for a VM or VMs, consider the following:

  • To use storage live migration to move a VM, you must disable changed block tracking on that VM.
  • Disabling changed block tracking prevents your backup solution from taking incremental backups of the VDIs associated with the VM. To take another set of incremental backups, you must enable changed block tracking again.
  • Changed block tracking cannot be enabled again by using XenCenter.

To disable changed block tracking, complete the following steps:

  1. In the left panel, select the VM or VMs that you want to disable changed block tracking on.
  2. From the main menu, select VM then Disable Changed Block Tracking.
  3. In the confirmation dialog that opens, select Yes to continue.

Viewing the changed block tracking status for a VDI

You can see whether changed block tracking is enabled for a VDI on the Storage tab for an SR.

If the SR is part of a pool where the changed block tracking feature is available, XenCenter displays the Changed Block Tracking column. This column shows whether changed block tracking is Enabled or Disabled for a VDI.