How many servers will we need for a certain application?
This document provides some insight into the servers necessary for WTS (Windows Terminal Services) or VDI (Virtual Desktop Infrastructure).
In the old days, companies had just a single mainframe and certain applications running on it. As CPU load generally is the primary bottleneck, the people responsible for the mainframe just looked at the CPU utilization and, when it was over 80%, they knew they had to buy a bigger one. When paging was too high they had to buy more memory.
Today we can run applications in many ways, and these ways all have different needs for computing power and memory.
First we look at desktops. As Windows Vista has been available since early 2007, most users found out running Windows Vista with less than one gigabyte of memory is quite slow.
When we have desktops, these should share their data. This is typically done over a file server. I have heard people say a modern server can handle 5,000 simultaneous users when this server is used as a file server.
How about Terminal Services? At the moment of this writing, most terminal servers are used by around 50 users. 32-Bit operating systems are mostly used; the problem with 64-Bit operating systems (Windows) is that not all drivers are available in 64-Bit versions. As companies have more users, load-balancing is used to connect all the users to multiple servers.
But Windows Server 2003 and Windows Server 2008 can handle many more concurrent users. In the paper on
Microsoft says they have tested several hundred users on hardware dated 2003. As today we have more advanced hardware with fast CPUs, many cores and lots of cheap memory, I believe running terminal servers with hundreds of users is no problem. Of course, 64-bit hardware and software is needed in this case.
But some key points should be kept in mind: Windows, as well as other modern operating systems, does paging as a default configuration. Paging means, memory not frequently used is written to the page files on disk, so less memory is needed. What happens if a machine has enough memory so that no paging is needed? As Windows always tries to have some memory space available for when new applications are started, it still writes pages to disk and frees the internal memory as it is not needed. The problem today is that the gap between CPU speed, memory speed and disk speed always widens. And as we have more memory, the page file on disk gets bigger and it takes even longer to read in the requested memory pages when needed.
We have a big WTS in use for our developers, running MS Visual Studio and Eclipse. Our WTS is sometimes up for many months and users do not log off. In the evening they disconnect their RDP session. When they come in the next morning they start their RDP client and can continue to work where they left off the day before. Our users find that straight-forward. When we had paging enabled, and when applications had not been used for maybe several days, the users clicked on the icon in the task bar and had to wait maybe 10 minutes until the application was paged in and the users could work again. Sometimes the application did not show up any more, even after hours. All these problems disappeared when we turned off paging in the Windows operating system. Without paging, our system always immediately responds to every action users make. It's a great system!
I can give the following advice: Buy so much memory for your servers that paging can be disabled. Memory is cheap and will get even cheaper. Disable paging by configuring Windows accordingly.
As you have no more paging, you do not need fast disks any longer. It is sufficient to buy big, cheap SATA disk drives instead of buying the faster but smaller and more expensive SCSI disks.
Let us talk about VDI or desktop virtualization. Using VDI means you have servers running virtualized single-user operating systems such as Windows XP or Windows Vista.
When VDI is used, multiple copies of the (guest) operating system are simultaneously in memory, so much more memory is needed. Remember that you should not have less than one gigabyte for every Windows Vista. And remember you should have paging turned off. When running VDI, I recommend you disable paging on the host level, but paging on the guest level will still be needed (otherwise you would need just too much memory). If you have a Windows Vista system and give it only one gigabyte of memory without paging, you will not be able to start many applications.
So when we have a server with 16 gigabytes of memory, we can only have around 16 simultaneous Windows Vista guests running.
So, compared to WTS, VDI needs much more hardware resources. However, there are still occasions where VDI is more suitable than WTS. And hardware keeps getting more powerful and cheaper, so VDI, in some cases, becomes more attractive.
13.08.08 Klaus Brandstätter
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