I haven't used Virtuozzo, but have used VMware extensively. From what I can tell of Virtuozzo, UML (User Mode Linux) is very much like Virtuozzo.
UML takes the normal Linux source code, makes a few modifications in a single patch, and then compiles it as a normal program. The result is a Linux kernel that can be run as a normal program and as a normal user.
The UML process will start a complete virtual Linux environment, with its own virtual hardware resources, much like VMware or Virtuozzo. Unlike VMware, it works with standard Linux file system images.
The original impetus was for development and debugging of kernel code (much like VMware is promoted as a development tool). Quickly, people realized that the ability to offer virtual Linux environments extended well beyond the development cycle, and now people are using it:
- To offer dedicated virtual instances on shared commodity hardware to customers in a hosting environment.
- As "sandboxes" for applications that require elevated privileges but carry special security risks.
- As "sandboxes" for legacy applications that require older versions of a kernel and/or system libraries.
UML meets all of the same criteria that SWsoft markets as capabilities of Virtuozzo VE.
UML, unlike Virtuozzo or VMware, runs on almost every platform that Linux runs on--including x86, ia64, PowerPC, Alpha, and other platforms. Additionally, work is under way to make it possible to run UML as a process under different operating systems, enabling you to run a complete UML Linux virtual environment under Solaris or Windows 2000.
While Virtuozzo certainly has the advantage of commercial support, organizations that already have developed their own Linux expertise can probably leverage UML at a lower cost than licensing and implementing Virtuozzo. For those who actively work in the Linux support community, the e-mail and IRC support channels for UML might actually be more effective than a commercial support option from a company like SWsoft.
As SWsoft has pointed out, there are strong differences between Virtuozzo's architecture and that of VMware's, and pretty much the same would apply to UML when compared to VMware. I use both UML and VMware, and see them addressing different problems that I face as a system administrator.
I was a little concerned about the accuracy of some of the statements made about VMware by Ed Bugnion. If, in fact, VMware does share memory between instances, then it is useless to me. If he was referring to the copy on write filesystem mechanism, then that is a VMware feature that I am glad to use (and is incidentally a feature of which UML is capable).
It is worth noting that there are also open source alternatives to VMware's complete virtual machine model. The most notable projects in this field are Bochs and MOL.
Bochs, like VMware, can run many different x86-compatible operating systems, including Windows and OS2. There is also a Windows version of Bochs, and apparently Bochs will virtualize x86 hardware for non-x86 platforms--something VMware can't do.
MOL does much the same thing, only virtualizing PowerPC hardware, enabling users to run MacOS and MacOS X images on their PowerPC-based Linux machines.
Thornton Prime
