Changed Block Tracking, or CBT, is a mechanism that allows applications (such as backup software) to find out which blocks of a file have changed since a previous point in time.

If you consider a virtual hard disk that’s a terabyte in size, a differential backup would involve reading the whole file to see what had changed and only back up the blocks that have changed. Whether using direct-attached or network-attached storage, this full-file scan can take a long time, which increases already-tight backup windows.

CBT allows the backup application to seek to the parts of the virtual hard disk file that have changed and only read the changed blocks.

Unfortunately, in Windows Server 2012r2 (and earlier) with Hyper-V, there was no in-built support for CBT. So backup vendors (such as our friends over at Veeam) had to implement their own custom CBT infrastructure, which may or may not be compatible with the storage at the backend.

With Windows Server 2016, CBT (also referred to as Resilient Change Tracking, or RCT) has been standardised within Windows itself. Vendors such as Veeam are able to make use of it and it is more likely to work with a wider variety of storage arrays. Veeam 9.5 brings support for CBT/RCT with Windows Server 2016. With Tintri support for Windows Server 2016 announced and to be released to GA imminently, the combination of Tintri, Veeam and Windows Server 2016 will allow you to enjoy standardised Changed Block Tracking.

[Above image created and distributed by David Pacey under Creative Commons 2.0 License]

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