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Design Decision: Autoscale Design

  • Contributed By: Nitin Mehta Special Thanks To: Mayank Singh, Kevin Nardone, Harsh Gupta


The goal of the document is to help answer FAQs on Autoscale to achieve the best cost optimization. It provides guidance for configuring Autoscale for different admin use cases and their infrastructure and technical requirements. This document keeps evolving as more use cases and capabilities are introduced within Autoscale.

Use cases related questions

Can an admin configure Autoscale to first exhaust on-premises resources and only then on-demand burst to the cloud?

  • The Autoscale selective power management feature is useful here. The admin must tag the cloud workloads within the delivery group that will be used for burst. Then configure Autoscale to power manage only these tagged workloads. The admin can then use zone preference to prefer on-premises workloads (which are always powered on) for incoming requests. After this configuration, incoming requests will first be brokered to on-premises resources until they are fully used. Cloud resources will be powered on only after the on-premises capacity is used.

Can an admin use their on-premises resources for normal operations and Cloud resources only in a business continuity or disaster recovery scenario?

  • Use the preceding configuration

Can an admin configure Autoscale for users ramping up slowly / seasonal users / not worry about capacity planning upfront?

  • When an organization has users onboarding slowly and does not want to provision the entire capacity ahead of time, then the dynamic provisioning feature with Autoscale can help. This feature constantly evaluates the load on the delivery group and creates or deletes machines accordingly. An admin must define the high and low watermarks for load on the delivery group to create machines. Admins can optionally define the maximum number of machines to be provisioned and tags to be applied to these machines.

How can the admin of an university with varying schedules and lectures during the day configure Autoscale?

  • The admin must understand the capacity utilization and lecture schedule and feed it into Autoscale schedule-based scaling to step up and step-down capacity during the day. Autoscale allows admins to vary the powered-on capacity with 30 min granularity and allows for different schedules on different days. If a volatile workload is expected, it is advised to configure the capacity buffer to avoid users having to wait for machines to power on.

How can the admin of an outsourcing firm with agents logging in at fixed timings configure Autoscale?

  • Use schedule-based scaling depending on the shift schedules. Autoscale has 30 mins granularity. Admins can configure Autoscale to power on different workloads at separate times of the day. If the number of users is unexpected, then admins also define the buffer capacity. Some platforms can take several minutes to switch on machines so do factor that while creating these schedules.

How can the admin configure Autoscale if they are using heavy machine instances?

  • If the organization provides static desktops to its users, there might be a use case where the admin wants the machines to be started when the user clicks the desktop icon (say for knowledge workers who occasionally work on those large machines) but shut them down at a specific time of the day say at 6 PM. In this case the admin can define the property AutomaticPowerOnForAssigned to false.

How can an admin configure Autoscale to save storage costs apart from compute?

  • With dynamic provisioning machines can be created and deleted using Citrix Machine Creation Services. This provides extra storage cost savings by de-allocating the disk when the machine is not needed.

Infrastructure questions

How can the admin choose the number of reserved instances (RI) to buy?

  • Observe the capacity using data under Monitor > Trends > Machine Usage over a time period (that covers all variations such as seasonality, work force changes and so on.) in the environment. The minimum capacity that is powered on for this time period can be a good indication of the minimum reserved instances you might buy. Some other parameters that define this are:

    • Is there a fixed schedule say 9 AM - 5 PM? That schedule would mean that machines can be shut down 66% of the time and achieve close to reserved instances cost savings of ~70%.
    • More RIs than the minimum capacity usage can be bought depending on workload usage and savings. For instance, say if there are 10% more machines running more than 50% of the time.
    • The organizations expansion plan must also be factored in.
  • Using Reserved Instances can save up to 70% costs compared to pay as you go machines over a three year duration.

How can the admin choose the number of burstable instances to purchase?

  • For virtual desktop users with high memory consumption (For instance applications such as Chrome browser, Office applications) but low CPU consumption, admins can consider using burstable VMs. These VMs might be half the cost of their peer machines (as is the case in Azure for B series and D series). In Citrix Director, check Resource utilization and average CPU consumption and match it with the base CPU% guarantee of the burstable VMs. It is recommended to use these machines for single session machines only. The reason for this is that the impact is limited compared to a multi-session machine where many users can be impacted if the CPU gets throttled.
  • This mechanism can be used to lower costs by ~50% from the regular pay as you go model.

How can the admin choose the right instance size?

  • Admins can optimize costs if they right size the machine instances in public clouds. Smaller instances host fewer user sessions than larger instances. Therefore, Autoscale powers off machines faster because it takes less time for the last user session to be logged off, thus reducing costs. Citrix recommends provisioning smaller instances if they match workload performance and capacity requirements. An effective way to understand this would be to have no more than 15-20 sessions per machine.

What scale limits can the admin operate under?

  • It depends on the limits of the chosen cloud platform and the provisioning technique. For example, if the cloud platform is Azure and the provisioning technique is MCS (Machine Creation Services) then the guidance is ~1200 machines per subscription.
  • It is also recommended to evaluate other limits like # of machines in a resource location and hosting connections. Visit the Limits page for more info. This is an evolving page.

Technical questions

How can the admin start new machines based on session count?

  • Look at the load index. By default, it is based on session count (250) and is the factor considered for powering on more machines.

How does Autoscale turn off machines?

  • Autoscale always attempts to scale down the number of powered-on machines in the delivery group to the configured pool size and capacity buffer. It does so by putting the excess machines with the fewest sessions into “drain state” and powering them off when all sessions are logged off. This occurs when session demand lessens, and the schedule requires fewer machines than are powered on. More info.

How does Autoscale turn on machines?

  • It powers on machines based on the schedule and load-based settings. If the current machines cannot accommodate the incoming load and respect the capacity buffer configured, then it will power on more machines. See this working example for a better understanding of how it works.

Where do I see machines which are candidates for shutting down?

  • Just like maintenance mode, a list of machines in drain state is maintained. If a machine is in drain state, then it is a candidate to be shut down. Once all the sessions are drained off, the machine is shut down. To find out which machines are in drain state, use the PowerShell command: Get-BrokerMachine -DrainingUntilShutdown $true. More info

How do I see my cost savings?

  • The Monitor > Trends > Machine Usage page also displays the estimated cost savings achieved by enabling Autoscale in the selected delivery group. Both $ saved and % savings using Autoscale are displayed. These savings can be sent as monthly savings reports to higher management.

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