Re: [Xen-users] Xen Compared

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

Yo

> Can someone explain the differences between Xen and Virtuozzo/OpenVZ.
> What are the pros/cons of each?

I work on Xen, and I get paid by XenSource for some stuff, so feel free to 
treat me as a bit biased ;-)  I don't know so much about OpenVZ and Virtuozzo 
but I'll have a go...

Xen

Pros:
- fast performance for paravirtualised VMs
- can run fully virtualised VMs (e.g. Windows)
- mix multiple operating systems on the same system
- can do live migration
- fairly strict partitioning of resources
- each guest has its own kernel - kernel exploits don't allow a guest to break 
out of its container.  Guests can run custom kernel code if necessary.
- supported by some distros (RHEL / CentOS 5 have out of the box support, for 
instance)

Cons:
- can be fiddly to install and manage - particularly if your distro doesn't 
have (good) support
- binary-only drivers may not work straightforwardly (if at all)
- "heavyweight" virtual machines, use a bit more resources, lower performance 
than "native" execution (usually not much lower, depending on the workload)
- OS support for paravirt mode is a bit patchy (Linux 2.6 support is good, 
NetBSD runs on 32-bit Xen, although I don't know if it runs on PAE.  Ditto 
Plan 9.  FreeBSD port stalled)

OpenVZ (and I guess Virtuozzo too, I don't know anything about it other than 
it's based on OpenVZ)

Pros:
- fast performance - should be pretty much as fast as running without 
virtualisation
- efficient resource usage - virtual machines memory allocations can be very 
flexible, according to load
- easy to administer
- should be easier to get binary drivers working

(- I heard suspend / resume and live migration was in development a while back 
but don't know the status)

Cons:
- less well supported by distros?
- lower resource isolation, a counterpoint to the more efficient utilisation
- kernel bugs seem "more likely" to allow a virtual machine to break out of 
their container and compromise the whole host
- limited to Linux-on-Linux and same kernel version for all VMs

There are doubtless things I've forgotten or not thought of.  I think the take 
home point is that they're both useful for different reasons.  I hope one day 
to see distributions shipping with support for both forms of 
virtualisation....  OpenVZ for lightweight partitioning, Xen for supporting 
heavierweight virtual machines when they are needed.

For comparison, OpenVZ is rather like Solaris Zones.  Sun are working on Xen 
support even though they already have Zones - they're both useful in 
different ways.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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