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Update: Virtual server backup

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Changed Block Tracking

Almost every backup application designed to protect virtual servers, especially those running VMware, supports Changed Block Tracking (CBT).

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With CBT, instead of backing up the entire VM -- or even just its changed files -- each time a backup job is initiated, only changed blocks within the VM are backed up. CBT dramatically reduces the amount of data that needs to be sent across the network to the backup server or backup target.

Typically, the process starts with the virtual server backup application performing a snapshot of the backup device so the previous backup state can be preserved. When the CBT backup occurs, the previous backups of the VMs on the backup device are updated with the changed blocks. Once the CBT transfer process is complete, the copy on the backup device represents the most recent backup data with previous backups stored as snapshots for point-in-time recoveries.

Granular recovery

In the past, one shortcoming of a CBT backup was that to recover a file from the backup instance, the entire instance had to be restored first. In that regard, CBT is similar to image-based backups. However, unlike image backups, modern virtual server backup software applications can recover components such as individual files and email messages from within the image.

Granular recoveries are typically done by mounting the VM directly from the backup devices and copying out the data, or via a recovery wizard that can peer into the VM and extract individual components. The recovery wizard method is preferred as it involves fewer steps and can be done from a single interface.

This was first published in September 2012

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