Customers considering server virtualization can be overwhelmed by having to research multiple vendors and, in some cases, multiple products from a single vendor. Evaluating all the options takes a lot of time, but selection shortcuts conclude in failed projects.
I've seen simple projects fail to show return on the investment because the adopted solution's cost of ownership was too high. Likewise, I've seen companies dump products that worked well in small projects because the product couldn't scale. Obviously, you want to avoid these situations. To help out, I've done some of your homework for you by evaluating the two main platforms of market leader VMware: free VMware Server and the pricey ESX Server.
Is a free product reliable enough?
Newcomers often think that because it's free, VMware Server has less features or is less reliable than its commercial counterpart. Nothing can be further from the truth. VMware Server is more than capable for serious and complex virtualization projects.
For years, VMware sold GSX Server with an enterprise-level price. At the GSX Server launch in 2001, the company touted its mainframe-class control and production-level features. About 300 companies worldwide participated in the its beta program during the previous year.
GSX Server was designed for data center deployment, as is ESX Server. When the VMware enterprise management tool, VirtualCenter, came out, the company was immediately able to control both products in the same way.
After releasing its first free virtualization product at the end of 2005 (VMware Player), the company decided to transform the upcoming version of GSX Server 4.0 into a free product too, renaming it Server 1.0.
GSX Server wasn't downgraded when VMware made it free of charge. It didn't become a different, less reliable product. Simply put, the consumer who lacks confidence in the capabilities of GSX is misinformed.
Performance versus flexibility
Apart from the price, the first difference between VMware Server and ESX Server that every consultant or salesman would underline is in performance.
As of today, nobody has published a benchmark comparison between the two platforms running the same virtual machine on the same hardware. But from the way they are architected, many people can predict which product should perform better.
While VMware Server needs an underlying operating system to be installed on, its bigger brother ESX doesn't. ESX is a bare metal solution, as the industry usually calls it. Part of ESX Server acts as an operating system, booting the hardware and managing it to achieve all required virtualization tasks, in the same way an appliance would act.
This difference in approach reflects a deep difference in behavior. While Server's capabilities and performance is capped by the host operating system, ESX Server is designed to take out the most from available hardware and with the thinnest OS layer possible.
"Performing by design" means ESX Server can offer customers not only better performance, but also a better consolidation ratio (the amount of concurrently running virtual machines that can be safely allocated for each CPU core in the physical host). While VMware suggests keeping this ratio between two and four with Server, customers can reach four to eight with ESX.
These values are highly dependent on the virtual machines' expected workloads and on the hosted applications. A very busy ESX Server may not allow more than three virtual machines per core and a very light Sever could easily run 10 Web servers per core. But in general, these numbers can be considered as a good reference point.
ESX Server can directly control the hardware components, which can improve performance in other aspects. For example, VMware developed a special file system called VMFS to store virtual machines, which is faster and more reliable than the traditional multi-purpose file systems that modern operating systems offer. Unfortunately, when a virtual machine stored on VMFS needs to be migrated on another VMware product, its format has to be converted.
VMware Server cannot count on VMFS, and its performance depends on Windows and Linux file systems, but its virtual machines can be moved on the fly onto any computer with a burned DVD or a USB key, just like a standard folder.
But a bare metal approach brings severe limitations along with benefits. Because the product itself acts as an OS, only hardware that has its drivers included will be compatible. At the moment of this writing, VMware doesn't include drivers for local SATA disks in ESX Server, and customers adopting it have to choose between local SCSI disks or remote storage facilities, like NAS and SANs.
Bare metal pros and cons
Customers must also be aware that not only could some equipment pieces not be usable, but a whole machine may also be incompatible. VMware reserves the right to officially support only a limited number of systems on the market, so you cannot necessarily run ESX Server on all the hardware available in the data center and expect support for it.
The hosted approach of VMware Server instead relies on the underlying operating system for hardware support and drivers availability. Anything the OS can do, like connecting a remote iSCSI disk or driving a local tape backup unit, can be done by the Server virtual machines.
Software availability and support are also affected by this difference in approach. ESX Server has a limited number of installed service utilities. Newly installed applications sometimes do not work because mandatory libraries are missing, and customers are highly discouraged to add them because it could undermine the reliability of the whole system.
This reduces risks but can translate into administrative pains anytime VMware didn't implement a component that we needed.
In VMware Server environments, any program can be installed on the host operating system. Customers can achieve tasks like disk defragmentation or backup and performance monitoring or remote management with the tools of their choice.
In the next part, we'll see how differences between ESX Server and VMware Server affect the security and learning curve, how the enterprise management product VirtualCenter adds more diversity between the two products and how this diversity affects support.