Hyper-V disk types

There are four primary disks types you can create in Hyper-V.\

Fixed

A fixed VHD volume is created up front and has all of its space allocated at the time the volume is created. So, if you create a new disk volume and give it a size of 127 GB, that new volume will immediately consume 127 GB of disk space on the host. There used to be a day when fixed virtual disk files were much better performers than their dynamic counterparts. Those days are largely gone. Although users might enjoy a slight performance benefit from using fixed virtual disks over dynamic ones, the difference is not huge.

Pros

  • By using fixed disks, you can better estimate overall host storage usage and not have to worry about accidentally overprovisioning storage resources.
  • Virtual disk files are portable and easily copied from machine to machine.
  • For virtual disks, these are the fastest, although the performance margins continue to close as Microsoft makes improvements in Hyper-V and VHD handling.

Cons

  • You waste a whole lot of storage if you make volumes too large.
  • You can’t shrink a fixed disk to make use of the “unused” space.

Dynamic

Like a fixed disk, you specify the size of a dynamic disk at the time its created but the amount of space on the host actually used by the disk depends on how much data is being stored on that particular volume. This concept is also known as thin provisioning.

For example, if you create a 127 GB dynamic disk but the virtual machine is using only 20 GB of space on that disk, which will also be the amount of disk space consumed by the volume. Whenever reasonable, I highly recommend that you use dynamic disks as your storage medium of choice.

Pros

  • You can stretch your expensive storage resources much further and still provide virtual machines with space allocations that make sense. This provides a lower total cost for the service.
  • Virtual disk files are portable and easily copied from machine to machine.

Cons

  • You can accidentally overprovision storage on the host and create infrastructure stability issues as a result.
  • Requires an additional level of oversight so that storage resources aren’t overprovisioned.
  • There is a slight performance hit as space is allocated for usage.
  • If you expand a volume, write operations may suffer during the expansion operations, but this is temporary.

At a former employer, although we used VMware, this thin provisioning concept follows and we use it for every single virtual machine we run as a way to reduce overall storage costs. We do carefully watch storage and migrate virtual machines to new stores as necessary, but it has helped us reduce the overall amount of storage we need for our virtual environment.

Pass-through

Sometimes, a service needs direct access to physical disks and won’t support the use of a VHD. Or, the service simply needs so much disk space that presenting a physical volume makes more sense. For example, while I was creating my Data Protection Manager 2010 course for Train Signal, I needed to use a storage volume that was huge in size and based on physical – not virtual – disks.

A pass-through disk allows you to mount a physical volume to a Hyper-V virtual machine. In my DPM course, I used a pass-through disk to act as the storage pool device that I needed to use for my lab scenarios. There are, however, some serious drawbacks to the use of pass-through disks.

Pros

  • Allows you access to volumes exceeding 2 TB in size from within the Hyper-V virtual machine.
  • Carries a performance gain over virtual disks since there is no abstraction taking place. The virtual machine has direct access to the disk.

Cons

  • Not portable at all. It’s tougher to move the storage to another server.
  • You cannot take snapshots of a pass-through disk.
  • The Hyper-V VSS writer cannot back up a pass-through disk. If you’re backup software uses this common protection technique, you’ll need to find alternative methods – such as installing a backup agent inside the virtual machine itself – for protecting the contents of the pass-through disk.

Differencing

A differencing disk is a dynamically expanding disk that is chained to a parent disk. With a differencing disk, the parent disk never changes; all changed blocks are reflected in the differencing disk. With this method, you can create a bunch of identical virtual machines and each can have its own differencing disk for customizations, but the parent disk is never changed.

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