![]() You cannot override this behavior! Standard and Production Checkpoints Note: backup software operations will always create differencing disks in the same location as the parent. If it doesn’t, you have a problem and will need to talk to your backup vendor. When the backup is complete, Hyper-V deletes the checkpoint and merges the differencing disk that it created back into its parent. While the disk and the state of the virtual machine are frozen in the checkpoint, the backup application can copy the contents without fear that they’ll change. When a Hyper-V aware backup application targets a virtual machine, Hyper-V will take a special checkpoint. Backup Softwareįor most of us, backup software is the most likely source of differencing disks. There are four generic methods by which differencing disks are created. In fact, most Hyper-V administrators will never directly create a differencing disk at all. Unlike fixed and dynamically expanding virtual hard disks, you don’t simply kick off a wizard and create a differencing disk from scratch. 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. 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
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