Citrix Provisioning Services, which evolved from their acquisition of the Ardence technology, enables some great concepts:

• Since the first time a Citrix customer deployed more than one WinFrame server, we’ve struggled with the issue of change control – how do we insure that, over time, all of the servers that are supposed to be identical do, in fact, remain identical? Booting and running them all from a single, read-only image is a great way to do that.
• It gives you an “undo” option when you upgrade your server image. You can make a copy of your read-only image, set it to read/write, apply your patches, updates, etc., reboot one server from the new image, do your testing, then set the new image to read-only, reboot your servers, and ba-da-boom ba-da-bing (that’s a technical term), in the time it takes them to reboot, they’re all running from the new image. If you then discover that there’s something wrong with the new image, point them back at the old image and reboot them again, and, in the time it takes them to reboot again, you’ve just rolled back to the old image.
• In a VDI scenario, not only do you enjoy the first two advantages, you also save a ton of expensive SAN storage. If your typical desktop image is, say, 10 Gb, and you want to deploy 100 virtual desktops, with some vendors’ approaches you will consume a full terabyte of expensive SAN storage. By using provisioning services, you consume only the 10 Gb required by the common image.

Unfortunately, when you convert a modern Microsoft OS image to a shared read-only image, it looks like a hardware change to the OS, and breaks the license activation. This is the case with Windows 2008, 2008 R2, Vista, and Windows 7.

Enter the KMS server. KMS stands for “Key Management Service,” and it’s one way to automate the activation of Microsoft volume licenses within an organization. There’s a pretty good video that you can download from Microsoft Technet that walks through the process of configuring a KMS server to automatically activate servers and workstations, but it was made prior to the release of 2008 R2, so it omits a very important point (which we will get to in due time).

The concept is that as an un-activated copy of Server 2008, Vista, or Win7 boots, it queries Active Directory to see if there is a KMS server on the network. If there is, it contacts the KMS server for activation. However, for reasons that are not at all clear to me, the KMS server must be contacted by a minimum number of machines before it will actually activate anything. So, each time a different machine contacts the KMS server for activation, it is assigned a unique ID number, and the KMS server increments its counter by one. When it has been contacted by a total of five different systems, it will begin to activate servers. When it has been contacted by a total of 25 different systems, it will begin to activate workstations.

Before the release of Server 2008 R2, only physical systems would increment the counter – virtual systems would not. (Don’t ask me how the KMS server could tell the difference – that’s one of the ongoing mysteries of KMS.) And that’s the message you’ll hear when you watch the video referenced earlier. However, if KMS is running on a Windows 2008 R2 server, both physical and virtual systems will increment the counter. Note also that what matters is the aggregate number of all systems that have contacted the server for activation, regardless of whether they’re running Server 2008, 2008 R2, Vista, or Win7.

If the threshold has not yet been reached, the system will not be activated, but will still run…within the constraints of the built-in 30-day “grace period” for activation. (Although the nag messages get pretty intrusive in the last three days of the grace period.) This, by the way, is good news if you’re looking at an evaluation or proof of concept that will involve fewer systems than it takes to meet the threshold – you should be OK as long as the evaluation term doesn’t exceed the 30-day grace period. The system will continue to check back in with the KMS server ever two hours to see if the threshold has been met. When it is met, all of the systems that have been waiting will be activated. Once activated, a system will attempt to check back in and renew its activation every 7 days. It must renew its activation within 180 days, or it will revert back to an un-activated state.

The KMS server keeps track of the ID numbers of the systems that have contacted it for activation. If an activated system does not check back in within 30 days, its ID number is removed from the KMS server’s cache, and the counter is decremented. If the count falls back below the threshold, the KMS server will stop activating systems. To help guard against this, the KMS server’s cache size is set to 2x the threshold. In other words, if you’re only activating servers, the cache will contain the IDs of the last 10 servers that have contacted it for activation. If you’re activating workstations, or a combination of workstations and servers, the cache will contain the IDs of the last 50 systems that have contacted it for activation.

The KMS service can be co-hosted with other services in your server infrastructure – you do not have to dedicate a server to this function. In fact, if all you care about are workstations, you can host the KMS service on a Win7 workstation. You’re going to want to have more than one KMS host running, to insure that it doesn’t become a single point of failure in your infrastructure. And remember, unless you’re going to be activating enough physical systems to meet the KMS threshold, you need to be running KMS on Server 2008 R2. That will give you the ability to activate “any Windows operating system that supports Volume Activation,” (which today means the four operating systems we’ve been discussing here), and count both physical and virtual systems toward the required threshold.

So…wrapping back around to the beginning of this discussion, if you want to use Provisioning Services to provision XenApp servers on Server 2008 (and remember, XenApp does not yet work on 2008 R2 as of this writing), you’re going to need a couple of KMS servers. And unless you have five or more physical 2008 servers that it can activate, you’re going to need to have your KMS servers running on R2. And even then, you’re going to need a total of at least five machines to meet the threshold before KMS will activate anything.

Likewise, if you want to use Provisioning Services to provision Win7 desktops – and I’m ignoring Vista here, because, even though I personally liked Vista, I think Win7 is sufficiently superior that it just doesn’t make sense at this point not to go to Win7 – you’re also going to need a couple of KMS servers. And unless you have 25 or more physical systems (in aggregate, counting both servers and workstations), they’re going to need to be running on R2. And in any event, you’re going to need a total of at least 25 systems.

For more information on exactly how KMS works, I strongly recommend the Technet Volume Activation Planning Guide for Windows 7 and Windows Server 2008 R2. Happy provisioning!