Overview of VMware Horizon Composer
Horizon Composer is used to provision linked clone virtual desktops, which are a type of virtual machine that shares a common virtual desktop master image, sometimes referred to as a golden image. Horizon and vSphere support up to 4,000 desktops for each single replica of the virtual desktop master image, which enables significant storage savings over traditional full clone virtual desktops.
The concept behind a linked clone desktop is demonstrated in the following diagram, which shows the relationship between the master Replica Disk Read Only and the Linked Clone Disk.
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This diagram is to illustrate the concept of a linked clone, the actual architecture of a linked clone virtual machine is explained next.
When a pool of linked clone desktops is provisioned, a replica of the virtual desktop master image is copied to storage accessible by the Horizon ESXi servers. This replica will be used as a read-only copy of the virtual desktop master image; all writes are redirected to unique disks that are attached to each linked clone. Linked-clones are provisioned using thin virtual disks, the configuration of which varies depending on the desktop pool settings.
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Beginning with Horizon 6.2, Composer gained the ability to create linked clone Windows RDS servers. Much of the information or language contained in this chapter will refer primarily to linked clone desktops, but understand that Composer is also required if you wish to use linked clone Windows RDS servers, and both use the concept of a shared replica.
The following diagram shows the configuration of two linked clone virtual desktops that share the same virtual desktop master image.
In this diagram, each Linked Clone Desktop is configured with optional disposable disks and persistent disks. Chapter 10, Creating Horizon Desktop Pools, explains the different options that affect the configuration of these and other linked clone disks.
When the linked clone desktops are powered on for the first time, they will begin redirecting any writes to the linked clone disks and will also create a virtual swap (vswp) file of a size equal to the amount of unreserved virtual RAM (vRAM) allocated to the virtual machine.
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Controlling the growth of a linked clone desktop requires an understanding of multiple topics, all of which will be described within this book. The following chapters contain valuable information that can help you control the storage utilization of linked clone desktops:
- Chapter 8, Implementing VMware User Environment Manager
- Chapter 10, Creating Horizon Desktop Pools
- Chapter 12, Performing Horizon Desktop Pool Maintenance
- Chapter 13, Creating a Master Horizon Desktop Image
Horizon Composer works at the direction of the Horizon Connection Server to provision and manage linked clone desktops. Horizon Composer uses the vSphere application program interface (API) to initiate whatever tasks are required based on the operation that is being performed. To achieve this, Horizon Composer requires specific permissions within vCenter Server, as well as permissions within Microsoft Active Directory, both of which will be described later in this chapter.
Horizon Clients do not come into contact with Horizon Composer; its role as an orchestration tool is to perform actions based on the configuration of the Horizon desktop pool. As such, Horizon Composer is depicted in the following architectural diagram as a stand-alone component that works directly with the vSphere and Horizon infrastructure.
The benefits of Horizon Composer do not start and end with gains in storage efficiency. The following features are just some of the ways with which you can leverage Horizon Composer to change how you manage desktop maintenance.
Refreshing linked clone desktops
The changes made to a linked clone desktop can be discarded on demand or upon user logoff. This operation is called a refresh, as it refreshes the desktop back to the exact configuration that it was in when it was first provisioned. User persistent data disks, if used, are not impacted by this action.
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Horizon also supports deleting the desktop as an additional option. After the desktop has been deleted, a new desktop is provisioned to replace it.
By refreshing desktops you maintain tight control over their storage utilization as all writes to the dedicated linked clone virtual disk are discarded. In addition, when all changes are discarded, the desktop is returned to the same state it was in when it was provisioned, which enables tight control over the end-user computing experience.
The refresh process is described in detail in Chapter 12, Performing Horizon Desktop Pool Maintenance .
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Refer to Chapter 8, Implementing VMware User Environment Manager , and Chapter 10, Creating Horizon Desktop Pools , for more information about how to preserve user persona data during recompose or refresh operations.
Recomposing linked clone desktops
A virtual desktop recompose operation is used when you need to update the master desktop replica image. One of the key methods of maintaining the storage efficiencies of linked clone desktops is to control the amount of data that is written to the linked clone disks, which if left unchecked will grow in size during the course of regular use. Traditional per-desktop administrative tasks such as installing applications and applying patches can quickly increase linked clone storage utilization, as the writes would be made to the linked clone disk of each desktop that was the target of the action. In addition, these operations would need to be repeated on each desktop, which can require significant management overhead.
A recompose operation replaces the linked clone replica disk with an updated version that uses the same operating system. Recomposing to a completely different operating system is not supported. During the recompose process, all virtual desktops will be provisioned again and linked to the new replica disk. If the desktop was configured with a persistent disk for storing user profile data—an option for dedicated assignment desktop pools—that disk would be retained and attached to the recomposed desktop.
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Technically speaking, a recompose operation requires a refresh operation. Either of these operations will temporarily reduce the amount of vSphere datastore capacity your persistent Horizon desktops are using.
The recompose process is described in detail in Chapter 12, Performing Horizon Desktop Pool Maintenance .
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When a desktop is recomposed, it will maintain the same computer name but will obtain a new DHCP lease. Your DHCP server must have a large enough pool of addresses available to handle these requests. In addition, it is recommended you use a short address lease period so that the now unused leases are quickly removed and the associated addresses are made available for use.