Question: Why are servers installed in VMs?

pparks1 · 16 Sep 2011

I just looked at my environment, and in our US production environment we have 6 physical servers (running VMWare vSphere 4.1), and hosting 70 virtual machines. In London, it's 3 Physical servers (running vSphere 4.1) and hosting 47 VM's. And our staging environment has 5 physical servers, hosting about 40 VM's. About 95% of our servers are Linux based, the remainder are Server 2003.

So, on 14 physical boxes, we run nearly 160 concurrent virtual servers. And we have plenty of room for more.

rraod · 16 Sep 2011

True... I agree with pparks1 on the advantages of rolling out VM servers in an enterprise.

Here I would like to add one important quality of VM servers. True portability....

The VMWare products VMWare Server, VMWare Workstation, VMware player are installed like an application on a host operating systems like windows or linux. Even though the host OSs are installed on different x86/x64 architecture machines, VMWare provides a standard x86/x64 virtual bios emulation to a VM.

VMware ESX server is based on linux and directly installs on the hardware without the need of a host OS. As one layer of host OS is made redundant, ESX server is fast and more efficient. This server also provides same x86/x64 virtual bios emulation to a VM.

Once a VM is created on VMWare product, all it needs is a standard set of VMWare tools (one set for each guest OS) providing the standard mouse, keyboard, video, audio, usb, SCSI, IDE and CDROM/DVDROM drivers.

So a VM created on a VMWare platform on a HP machine with different hardware components, will work straight away when moved to an IBM server or for that matter any Intel/AMD architecture machines with hardware components from different vendors.

Another feature is that VMWare supports any number of VMs on a single host machine (only limitation is on hardware resources). So a Windows VM, a Linux VM or a unix VM (ex. Solaris on x86) can co-exist on the same x86/x64 machine simultaneously.

preiius · 16 Sep 2011

Great thread. I don't want to spoil the party, but is there anything that's not suitable to run in a virtual machine, e.g. database server?

rraod · 17 Sep 2011

Any application which directly needs to access the hardware without going through the OS layer will suffer in a VM environment. For ex: Games.

VMs are not good for playing games... as there are three extra layers between the games and the hardware. Host OS, VMware application and the Guest OS.

The graphics in VMWare have a generic driver. As of now the VMWare supplied graphics driver can not be replaced with a fast one. So the limitation on graphics will affect the graphic intensive applications.

Known limitations of VMware ESX server, as of May 2009, include the following:

Infrastructure limitations
Some limitations in ESX Server 4 may constrain the design of data centers:

Guest system maximum RAM: 255 GB
Host system maximum RAM: 1 TB
Number of hosts in a high availability cluster: 32
Number of Primary Nodes in ESX Cluster high availability: 5
Number of hosts in a Distributed Resource Scheduler cluster: 32
Maximum number of processors per virtual machine: 8
Maximum number of processors per host: 160
Maximum number of cores per processor: 12
Maximum number of virtual machines per host: 320
VMFS-3 limits files to 262,144 (218) blocks, which translates to 256 GB for 1 MB block sizes (the default) or up to 2 TB for 8 MB block sizes. However you should be aware that on a VMFS Boot drive, it is very difficult to use anything other than 1 MB Block size.

Performance limitations

In terms of performance, virtualization imposes a cost in the additional work the CPU has to perform to virtualize the underlying hardware. Instructions that perform this extra work, and other activities that require virtualization, tend to lay in operating system calls. In an unmodified operating system, OS calls introduce the greatest portion of virtualization "overhead".

Paravirtualization or other virtualization techniques may help with these issues. VMware invented the Virtual Machine Interface for this purpose, and selected operating systems currently support this. A comparison between full virtualization and paravirtualization for the ESX Server shows that in some cases paravirtualization is much faster.

VMWare is most suitable for Enterprise solutions and in Data Centers. Database Servers can be run more effectively on VMs. See the links below.

Virtualizing Microsoft SQL on VMware vSphere

Oracle: Database on VMWare vSphere

VMWare is one of the better virtualization platforms around, and is quickly replacing full hardware dedicated servers.

For more information on VMWare please visit the VMWare site.