Differencing Disks in Hyper-V

A differencing disk is type of virtual disk which is used in cases where one wants to isolate the changes made to guest OS or the virtual hard disk attached to that particular virtual machine. The main thing which required for this differencing disk is to have the disk to be associated to exist before creating one. Where one can you inspect disk option to check the associativity of the disk to its parent disk or you can use reconnect option (all these in Hyper-V environment) hence making differencing disk ready to store all changes which would have been stored in parent disk which makes sure of no alterations to parent disk. It stores data dynamically as data is written to it and can grow as large as the maximum size allocated space for parent disk while creating it.

Content stored in these disks can be merged with parent disks if required but this alters parent disk and merging it to new virtual disk helps retain both of them with contents of both the disks. Which means many differencing disks can share one parent disk and have their own differencing disk.

This feature is particularly useful where you have many virtual machines which can share same parent disk which can store operating system and differencing disk storing the configuration that is different from parent. We have to be careful in locking the disks other than most recent child disk making child disk writable which helps in validating the chain of differencing disks in those consecutive versions.