Citrix MCS: What Is It and How Does It Work?

What Is Citrix Machine Creation Services (MCS)?

Citrix MCS is one of the many components of the Citrix Virtual Apps and Desktops infrastructure platform that enable IT administrators to design and deliver virtual desktops.

How Citrix MCS Works

Using application programming interfaces (APIs), Citrix MCS configures, starts, stops and deletes virtual machines (VMs) through the underlying hypervisor. This is done by utilizing a master virtual machine (VM) to create copies—also called linked clones—to provision virtual desktops in a one-to-one ratio (one image per one user). The clones include an identity disk that connects back to the master image; a differencing disk is used to separate the VMs from each other; and, optionally, a personal vDisk, Citrix’s dedicated virtual disk stores users’ files, settings and other data

Deployment Options

IT admins can create three types of virtual desktops through Citrix MCS:

Pooled-random desktops

Virtual desktops are assigned to users randomly (non-persistent) every time they request a virtual desktop infrastructure (VDI) session. Any user-specific changes on these desktops get erased on each reboot. Useful for quick access to specific desktops that could be used by either employees or guests (i.e., event booths/kiosks).

Pooled-static desktops

Virtual desktops are assigned to a specific user who is the only person able to use that desktop. Still, the user’s data and settings do not carry over from session to session. Ideal to provide guaranteed access to specific users no matter the workload.

Dedicated desktops

Virtual desktops are assigned to specific users (usually IT admins or high-position employees). They include a personal vDisk to save any changes made on the desktop. These are essentially PCs stored in the server.

Building Catalogs for Citrix MCS

To use Citrix MCS, the IT admin first needs to go through a time-consuming process to create machine catalogs. A machine catalog is a group of pooled virtual or physical machines that are created in an IT infrastructure. IT admins build VMs and appoint them as master VMs utilizing the Citrix Studio management console. Citrix MCS creates a snapshot of the master VM and a full copy of the snapshot, which serves as the master image, then adds the number of desktops/user admins requested to Active Directory to merge them under the same network. It is a lengthy process that could be avoided to optimize a virtual solution deployment.

To make changes to the virtual desktops, IT admins can update the master VM and then select the update option in Citrix Studio. To update pooled-random and pooled-static desktops, Citrix MCS creates entirely new clones to replace the old ones every time a user boots up a desktop. Dedicated desktops cannot be updated without erasing all user customizations first.


MCS isn’t the only option you have for configuring and delivering virtual desktops to users. Citrix offers Provisioning Services (PVS) as an alternative to MCS. While both have the same goal of centralizing VM management, each takes a different approach to VDI delivery. Machine Creation Services uses hypervisor APIs to create copies of linked clones from an underlying master VM.

On the other hand, PVS leverages a software-streaming technology that configures and delivers patches and updates to multiple VMs via a shared desktop image. So, which provisioning model is appropriate for image management tasks? Answering this question is tricky because of the complex logistics of meeting business and operational requirements.

The table below summarizes instances where each image management deployment is appropriate:

Decision factor Description Recommended model
Physical machines The task involves provisioning resources to standardized
desktops such as those in call centers, thin clients, and
lab environments.
PVS. Citrix does not support MCS for standardized desktop
Cloud deployment The task involves running virtual desktops and apps in the
public cloud such as Azure and Google Cloud Platform (GCP).
MCS. Technical limitations hinder PVS from running in cloud
Persistent desktops The project involves deploying virtual desktops in
persistent mode.
MCS. There are no operational or performance benefits when
using PVS for persistent desktops.
License entitlement The project involves provisioning resources to multiple VMs
without being hindered by license entitlements.
MCS. Citric does not incorporate PVS with Virtual Apps
Standard and Virtual Apps Advanced
Technical skills You are implementing the project for a company that is new
to the Citrix environment.
MCS. PVS requires extra technical and capabilities to
Familiarity with the model You are implementing VDI where the team is familiar with
one model’s technical aspects that meet their requirements.
Either MCS or PVS.
Complex multi-site architecture You are implementing VDI in environments where replicating
desktop instances across multiple datastores is crucial.
PVS. MCS is complex because you have to use PowerShell
Size of the environment You are implementing VDI where the scale of the target
environment is essential.
MCS is appropriate for smaller environments while PVS works
well with larger environments
Network bottlenecks You are implementing VDI in environments where the network
is a challenge.
Use MCS if bandwidth is a challenge.

Common Issues with Machine Creation Services

Deployment of virtual desktops with MCS can be a fairly complex process. Before embarking on application delivery with MCS, you have to consider storage capacity and solutions, among other factors. MCS/IO, a storage optimization feature in MCS, eases this process considerably, but much of the work still requires technical expertise. For example, you have to specify the number of VMs to create and the amount of memory to allocate to each VM. You also have to configure a cache for use with temporary data on each machine.

Aside from the complicated setup process, MCS features have been found to cause problems. One issue involved MCS/IO causing VMs to freeze during peak hours. The workaround was to redeploy the catalog without using MCS/IO, circumventing the advantages offered by the feature. In another instance, operating system failures resulted in MCS/IO catalogs with 32-bit VMs using 256MB or more in RAM cache. In this case, the solution was to allot less RAM cache to the VMs, again negating MCS/IO’s benefits.

Citrix releases fixes promptly enough, but there are also some issues that require workarounds because they arise from incompatibilities with some other system software or hardware. For example, MCS VMs have been known to fail when used with Nutanix hypervisors, cause access problems on Hyper-V, and lead to VMWare VMs containing ghost network interface cards.

Publishing Virtual Desktops with Parallels RAS

There is another alternative to consider when publishing desktops and applications—Parallels® Remote Application Server (RAS). Three great benefits include:

Download your free 30-day evaluation period of Parallels RAS and experience intuitive application and desktop publishing today!