Virtual Machine Storage Summary – Systems Engineering & Integration

When creating a new Virtual Machine (VM), the operating system and other contents of the VM are stored in a VHD file.

Keep in mind the following when creating a new VM:

When you create a VHD file during the process of creating a new VM, the VHD file will be a dynamically expanding disk.
To use a fixed sized VHD with a VM, create the VHD before you create the VM.
You have the option to copy the contents of an existing VHD or an existing physical disk to a new VHD.
VHDs on IDE require the VM to be powered off in order to make changes.
VHDs on SCSI are hot swappable, meaning that you can make changes to the VHD while it is running.

The following table describes management tasks you can perform on VHDs:

Action	Description
Expand	Expand allows you to increase the maximum size available in a VHD file.
Shrink	Shrink allows you to reduce the maximum size available in a VHD file.
Compact	Compact allows you to reduce the size of a dynamically expanding VHD. Dynamically expanding VHD files: Increase in size as files are added. Do not automatically reduce in size when files are deleted. The storage capacity of the VHD remains unchanged when you compact the drive.
Convert	Convert allows you to: Change the format of a file from a VHD to VHDX, or change the format from VHDX to VHD. Change the type from fixed to dynamically expanding or from dynamically expanding to fixed. When you convert a VHD file, it makes a new copy of the file. For the changes to take effect, copy the new file to the same location as the VHD file you changed.

Do not edit a disk with snapshots, replication enabled, or differencing disks.

Keep in mind the following regarding Hyper-V in Windows Server 2012:

You can replicate a VM to a VM hosted on another Windows Server 2012 with the Hyper-V role installed.
You cannot edit a running VHD.
You can clone VM domain controllers.

The following table describes features you can use with VMs:

Feature	Description
Pass-through disk	When you connect a physical hard disk to a VM, the hard disk is referred to as a pass-through disk. The physical hard drive must be on the virtualization server or it can be a network-attached disk. The pass-through disk must be offline so the VM can have exclusive access to it. The Disk Management snap-in cannot be used to add a pass-through disk to a VM.
Differencing disk	A differencing disk is a VHD associated with another disk and contains only changes to the associated disk. The differencing disk is referred to as the child disk; the disk it is associated with is the parent disk. The parent disk remains unchanged. The child disk contains the changes to the parent disk. Differencing disks save space but require more processing resources. A differencing disk must be the same format (VHD or VHDX) as the parent. A differencing disk can be merged with the parent using Edit in Hyper-V Manager. Differencing disks are not recommended for use in a production environment.
Snapshot/Checkpoint	A snapshot, also known as a checkpoint, can be used to restore a virtual machine to a previous state. You can apply multiple snapshots to get to an exact state of the VM. Space is reclaimed when you delete a snapshot. In Windows Server 2012, a Paused-critical status indicates that the host machine is out of space for the snapshots and has been paused to allow the snapshot to be redirected to another drive. Snapshots should be deleted through the Hyper-V Manager console. Merge a snapshot as soon as you determine the snapshot is the state to which you want to restore the VM. Large snapshots take time to merge. Windows Server 2012 allows you to merge a snapshot to a running VM. Merging improves disk performance. When you delete a snapshot, the snapshot is merged with the preceding snapshot and deletes subsequent snapshots. In Windows Server 2012, a domain controller is prevented from replicating old information when a snapshot is merged. In Windows Server 2012 R2, the Checkpoint-VM cmdlet is used to create a checkpoint on a VM. Keep in mind the following when creating a checkpoint: The Checkpoint-VM cmdlet lets you create the checkpoint while the virtual machine is running. The -AsJob parameter processes the cmdlet as a background job. The -PassThru parameter pipes the checkpoint. The PowerShell commands Export-VM and Export-VMSnapshot export a checkpoint.