In this post and accompanying video, we go over the benefits and considerations of using ephemeral disks with Azure Windows Virtual Desktop Session Hosts.  We review the performance improvements as well as limitations that come with ephemeral disks, including the inability to deallocate Session Hosts.  Next, we examine Azure Reserved Instances and how they can be used to keep the cost of Session Hosts down without using an auto scale solution.

There will always be a bottleneck, don’t let it be your infrastructure.  When it comes to Windows Virtual Desktop (WVD), you can take a few steps to minimize performance issues.  Using Azure NetApp or Premium Storage accounts for profile shares, keep all resources in the same region, and right-size Session Hosts are just a few steps to take.

Another option is to use ephemeral disks with WVD Session Hosts.  Ephemeral disk resides entirely in an Azure virtual machine’s cache.  A storage account does not back them, and, therefore, you do not need to pay for a managed OS disk.

To use a virtual machine’s cache for an OS disk, it must be big enough for the OS.  For example, a base Windows image takes up about 127 GB of space.  A Standard_DS2_V2 has 86 GB of cache, not enough to fit a Windows Install.  A Standard_DS3_V2 has 172 GB of cache, enough for a Windows install.

Ephemeral disks are only intended for stateless workloads.  The OS only exists in cache, and if the physical host ever went down, any virtual machines with ephemeral disks would need to be reimaged. Virtual machines with ephemeral disks cannot be resized or deallocated without also being reimaged.

Many organizations have used an auto-scale solution to deallocated virtual machines during off-peak times. It is not possible to deallocate virtual machines with ephemeral disks.  Instead, save money with Azure Reserved Instances.

A Reserved Instance is a commitment to purchase a resource for a 1- or 3-year term.  In return, the organization gets a deep discount on the cost of the virtual machine.  The chart below shows a comparison of 10 virtual machines at a pay-as-you-go rate, deallocated 40% of the time, and the same type and number of virtual machines with a 1- and 3-year reserved instance commitment.

Some organizations are reluctant to purchase Reserved Instances due to the commitment.  Here are some important points about Reserved instances:

• There are options to bill up-front or month-to-month over the term of the reservation.
• Reservations apply to virtual machine types, not specific virtual machines.  Reservations are scoped to a resource group, subscription, or billing context.
• There is flexibility in how the reservations are applied.  A reservation can be split between multiple, smaller virtual machines or apply to a fraction of a later virtual machine.

Check out the accompanying video for more details on increasing performance and lowering costs for WVD Session Hosts.