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Performance Evaluation of Xen Vs. OpenVZ
An anonymous reader writes "Compared to an operating-system-level virtualization technology like OpenVZ, Xen — a hypervisor-level virtualization technology that allows multiple operating systems to be run with and without para-virtualization — trades off performance for much better isolation and security. OpenVZ's performance advantage due to running virtual containers in a single operating system kernel can be significant. A performance evaluation study (PDF) done by researchers at the University of Michigan and HP labs provides insight into how big a performance penalty Zen pays and what causes the overheads (primarily L2 cache misses)." From the report: "We compare both technologies with a base system in terms of application performance, resource consumption, scalability, low-level system metrics like cache misses and virtualization-specific metrics like Domain-0 consumption in Xen. Our experiments indicate that the average response time can increase by over 400% in Xen and only a modest 100% in OpenVZ as the number of application instances grows from one to four... A similar trend is observed in CPU consumptions of virtual containers."
When you sit down and do your homework, and don't oversell as you said, OpenVZ gets the job done. I was looking for a good way of virtualizing a new server we were moving to and I've gotta say I can't see myself ever NOT installing a server in either an OpenVZ VE or a Xen domU ever again.... with OpenVZ you can give one VE (almost) all the resources of the hardware its running on, and when that one server outgrows that hardware doing a vzdump or vzmigrate is sooo easy.
The big difference between Xen and OpenVZ comes down to what openvz calls "privvmpages" - memory that is claimed by running processes but not actually used.
The example here is this: two OpenVZ virtual environments (VE) can be set up on a server with, say, 1 gig of ram, with a gig of swap underneath it (So, RAM+Swap equals 2 gigs).
Those two virtual environments can be "oversold" in the amount of privvmpages they're allowed to use, because processes ask for more memory than they _actually use_ all the time. So let's say we give those two VEs 1.5 gigs of privvmpages (total of 3 gigs - more than RAM+Swap), but we only give them each 500 megs of oomguarpages. (less than RAM+Swap).
The thing that _should never_ go over RAM+Swap is oomguarpages (out of memory guaranteed pages) - pages of memory that are guaranteed (OpenVZ measures some of its resources in pages and some in megs).
With Xen, on that server I just described you're locked in - there's no bursting, and there's no dynamic allocation going on. You give one domU 750 megs of ram and you give the other domU 750. That's it.
Here's to the crazy ones
Here are a few talking points based on my experiences with both Xen and VMWare (ESX Server & Workstation).
1. Cost - no contest, xen wins hands down $0 vs $5000/cpu.
2. Performance - xen wins noticably, i can get away with running 8 virtual machines with 1/4 the hardware that VMWare required for 6.
3. Capabilities - VMware versions ESX GSX and up beat Xen in ease of use and flexibility, anything less and Xen wins. Xen does have a quicker live migration capability, but falls short on conencting external hardware to the virtual machine (something that is trivial in VMware).
4. Stability - about even, maybe Xen. I've seen ESX crash once, and have never seen Xen crash.
5. Ease of use - VMWare no question. Theres a learning curve to Xen and setting up new VM images is a pain. (which is disappearing) There are fantastic tools for VMware that will let you manage virtual machine creation and even migration from a physical box. (some of which can be used to feed xen too =))
The bottom line. A Xen setup (using open source version) that can easily run 12 VMs costs about $2,000 on DIY hardware. A VMWare server/software combo to do the same? about $30,000. Assume an additional 40-80 hours of learning to get up to speed with Xen vs VMware.
The comments above cover the open source / free version of Xen. I have no experience with the commercial Xen offerings.
Personally i'm Xen biased due to cost.