Dell EMC Avamar for VMware 19.7 User Guide

Changed block tracking

Changed block tracking is a VMware feature that tracks which file system blocks on a virtual machine have changed between backups.

Changed block tracking identifies unused space on a virtual disk during the initial backup of the virtual machine, and also empty space that has not changed since the previous backup. Avamar data deduplication performs a similar function. However, using this feature provides valuable I/O reduction earlier in the backup process. Changed block tracking dramatically improves performance if SAN connectivity is not available.

If changed block tracking is not enabled, each virtual machine file system image must be fully processed for each backup, possibly resulting in unacceptably long backup windows, and excessive back-end storage read/write activity.

Changed block tracking can also reduce the time that is required to restore (“roll back”) a virtual machine to a recent backup image by automatically eliminating unnecessary writes during the restore process.

Changed block tracking is only available with the following types of virtual machines that use the following types of virtual disk formats:

• Virtual machine versions 7 and later

  The earlier virtual machine version 4 is commonly used on ESX 3.X hosts and in virtual machines that are deployed from templates that support both ESX 3.x and 4.0 hosts. The version of a virtual machine does not change when the underlying ESX host is upgraded. Many commercial appliances exist in version 4 to allow deployment on ESX 3.x hosts.

  vCenter version 4 provides the ability to upgrade version 4 virtual machine hardware from to version 7 virtual machine hardware. This upgrade is irreversible and makes the virtual machine incompatible with earlier versions of VMware software products. vCenter online help provides details.

• Disks cannot be physical compatibility RDM
• The same disk cannot be mounted by multiple virtual machines
• Virtual machines must be in a configuration that supports snapshots

Enabling changed block tracking does not take effect until any of the following actions occur on the virtual machine: reboot, power on, resume after suspend, or migrate.