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VMware Instant Clone technology can help IT further consolidate virtual desktops' resources to make the most of their performance.
One of the central themes of virtual desktops is sharing resources. Users on Remote Desktop Session Host share a Windows server. VDI users share a hypervisor host. Desktop as a service users share a cloud service. Even disk efficiency technologies are about sharing a master disk and only storing changes. All these technologies are about lowering the cost of virtual desktops through consolidation.
The latest sharing technique happens with the virtual machine (VM) RAM. VMware's Instant Clone desktops, for instance, share not just their disk capacity but also their RAM. This setup allows IT to provision virtual desktops even faster.
VMware now has three ways for its VDI product, Horizon View, to provision virtual desktops:
1. Deliver full clones using a template and customization specifications. Full clones share nothing with the parent VM, and IT must maintain them like any other Windows machine.
2. Deliver linked clones based on a snapshot of VM. Linked clones share the same read-only base disk and store their own changes. IT can maintain linked clones by updating the parent VM and recomposing the clones. Recomposing a 500 desktop pool, however, can take a couple of hours.
3. Deliver Instant Clone VMs, which use shared RAM as well as shared disks. These VMs are also based off a snapshot of a parent VM, and IT can update them, updating the original VM and pushing out a new VDI image. Updating each Instant Clone takes a few seconds, so a large pool will update in minutes rather than hours.
Full and linked clone VMs have their own RAM and CPU. Both types of VMs must go through at least one Windows startup sequence to be available to users. VMware Instant Clone VMs, on the other hand, do not need to start Windows before the new desktop is ready.
Shave me off a VM
The VMware Instant Clone feature originated in Project Fargo along with the idea that a new VM can be a copy of an existing, running VM. At first, the new VM is a thin sliver of resources, but over time it will grow. The new VM starts with the same disk and memory contents as the original VM. In fact, it uses the original VM's RAM and disk. The disk is like a conventional VM disk snapshot; it writes changes to a sparse file.
RAM uses a similar mechanism, but with the hypervisor's RAM rather than any kind of file. The new VM uses the original VM's RAM and only needs its own RAM allocated when it writes disk to memory. In my testing, the new VM does still have a significant RAM footprint -- around 50% of the configured memory. Still, Instant Clones save about 50% of RAM compared to a full or linked clone.
Plus, there is no waiting for the VM to boot. Within a few seconds, the new virtual desktop is available for the user to log on. In my testing, the new desktop was ready within 10 seconds.
Users are unique
The biggest challenge with VMware instant clone desktops is that they are ephemeral. When the user logs off, the desktop VM is destroyed. There is no persistence inside the VM. IT must find a way to maintain user settings between logins. This problem is the same one that faces all nonpersistent VDI deployments -- keeping users unique without having unique VMs. There are a lot of products, including from VMware and a number of third-party vendors, that make roaming profiles work properly.
Storage is still key
All this sharing does not remove the need for the resources that virtual desktops actually consume. IT still needs enough CPU capacity for the applications users run. Admins also need enough RAM, because instant clones only share the initial operating system RAM footprint. Once users start launching applications, each VM will consume RAM for those applications.
Storage will still be key to a good user experience. Each instant clone VM will still create a .vswp file when it powers on, so it will use disk capacity. Also, the clone rather than the parent VM will use most of the disk I/O, so the replica must be on fast storage -- ideally RAM cache or a solid-state drive. As with linked clones, the delta disks see a lot of write operations, particularly at login when the user profile loads. Fast storage will make a big difference to the responsiveness of Instant Clone virtual desktops from the end user's perspective, too.
VMware Instant Clone desktops allow even greater agility in desktop provisioning. Nonpersistent desktop provisioning and updating is fast. Updates are much simpler than linked clone desktops and they require fewer resources overall.
How to manage virtual desktop pools
Ways to automate VDI deployments
How VMware linked clones work