The normal tools that you have access to in Windows without Hyper-V cannot create a differencing disk, but you can mount one as long as its parent is present.
#Virtual disk aid driver#
However, the virtual disk driver is part of the Windows operating system. This is a Hyper-V blog, so I mostly only talk about Hyper-V. If the child does not have a record of any changes to the block(s), the virtual disk driver retrieves them from the parent. If it does, then the child provides the data for the read. When the virtual machine requests data from its disk, the virtual disk driver first checks to see if the child has a record of the requested block(s). If you’ve got that down, then reads are easy to understand. The child knows who its parent is, but that knowledge is not reciprocated. You cannot scan the file and discover that it has a child. The most important thing to understand is that the virtual disk driver makes a choice to write to the differencing disk. It tracks which block(s) in the original file were targeted and what their new contents would have been. When Hyper-V needs to write to a virtual disk that has a differencing child, the virtual disk driver redirects the write into a differencing disk. The concept behind the functioning of a differencing disk is very simple. Be aware that a differencing disk attached to a dynamically expanding disk does have the potential to outgrow its parent, if that disk isn’t fully expanded.
I say “root” because, even though a differencing disk can be the parent of another differencing disk, there must be a non-differencing disk at the very top for any of them to be useful. The maximum size of a differencing disk is equal to the maximum size of the root parent. For Hyper-V versions past 2008 R2, this operation can take place while the disk is in use Hyper-V can merge the change data back into the parent, destroying the differencing disk in the process.Any modification to the data of the parent of a differencing disk effectively orphans the differencing disk, rendering it useless.The parent of a differencing disk can be any of the three types (fixed, dynamically expanding, or differencing).You cannot attach them to pass-through disks, a file system, a LUN, a remote share, or anything else. The parent of a differencing disk must be another virtual hard disk.A differencing disk must have exactly one parent.The salient properties of differencing disks are: What are Hyper-V Differencing Disks?Ī differencing disk contains block data that represents changes to a parent virtual hard disk. Your Hyper-V knowledge cannot be complete without an understanding of the form and function of differencing disks, so let’s take a look. Administrators don’t deal directly with differencing disks as often as they work with the other two types, but they are hardly rare. There’s another type that enjoys significantly less press: differencing disks. Usually when we talk about Hyper-V’s virtual disk types, we focus on fixed and dynamically expanding.