When we size XenApp virtual machines for an on-premises deployment, we often talk about larger servers and NUMA. But what about sizing XenApp instances if we host in Azure?
Azure has an extensive library of instances, with each instance providing different amounts of CPU, RAM, storage and GPU options. If we run the same scalability test across many of these types of Azure instances, we end up with the following density numbers.
If we simply use these results as our guide, most of us would select the D13v2 instance as it gives us the greatest density. However, with cloud deployments, we need to did a little deeper.
Each Azure instance has a cost per hour. We need to divide this cost by the number of users the instance supports from our scalability testing. This gives us a price per user per hour
Quickly, we see that the A and D series instances cost the most, leading us to look at the Dv2 series as the cheapest option. Within the Dv2 series of instances, the D2v2 has a slight edge over other Dv2 instances. Take a look at D13v2. The price per user per hour is very close to the D2v2 instance, with only a difference of 2 cents per user per hour.
In an on-premises deployment, most of us would go with the larger sized instances (like the D13v2 instance) as this would cost less because we must include the costs of the Windows OS license; but in Azure, the cost of the Windows OS license is included in the instance cost per hour.
In addition, in an on-premises deployment, most of us keep all instances powered on at all times; but in a cloud deployment, we want to power off instances when not in use and power on when needed to help us keep costs low. Let’s see what happens to our costs when we scale up our Azure D2v2 and D13v2 instances with 1,000 users.
During scale-up, every time we power on a new Azure instance, we incur the full cost of the instance, regardless of the number of users. Because the D2v2 instances are smaller, we have more of them (67 in total), but the cost increases with a new instance powering on is smaller than the D13v2 (18 instances). As we continue to scale up to 1,000 users, the difference in costs over 24 hours between the D13v2 and D2v2 becomes greater.
But what about scaling down? In an effort to reduce costs, we want to power off idle instances (instances with no users). This gets even more interesting with XenApp because we cannot move user sessions between instances; we must wait for all users to log off of the instance before we can safely power off the instance. Unfortunately, users who log off are randomly distributed across all instances; preventing us from simply powering off enough to bring our total capacity down to 100 users.
What if in our 1,000 user scenario, most users (900 of them) would log off at 5PM. How many instances could we be able to power off because they are idle?
Because we have so many D2v2 instances to support 1,000 users (67 of them in total), when 900 users randomly log off, we would be able to power off 15 instances, on average because they have no users. But with the D13v2 instances, we only need 18 instances to support 1,000 users. When we randomly log off 900 users, each one of the 18 instances still has active sessions, preventing us from shutting down any instances.
The savings with the smaller D2v2 instances quickly adds up.
Over the course of 1 year, by using the smaller D2v2 instances, our Azure compute costs are over $40,000 lower than the larger D13v2 instances even though D13v2 has almost 4x the scalability of the D2v2.
When planning a cloud XenApp deployment, we must focus on cost and not size.
Daniel (Follow on Twitter @djfeller)
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