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  1. #1
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    OpenVZ, Xen and KVM

    Hi guys,

    Sorry for my newbie questions but I want to get a VPS and checked numerous offers. I found some VPS providers offer VPS servers based on OpenVZ, Xen (Pure and Paravirtualization) and KVM.

    I know about OpenVZ but what is KVM and Xen Paravirtualization ? I am asking because the price difference is huge between Pure Xen and Xen Paravirt but the Xen Paravit and KVM units cost like the identical OpenVZ unit. Why are the prices so different for Pure Xen and Paravit ? Is KVM better than OpenVZ and Xen Paravirt ?

    Thanks for the explanation.

  2. #2
    you guaranteed resources and virtually zero overselling with xen and kvm as it only allows resources to be allocated once with openvz the same resources can be offered to numerous vps's
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  3. #3
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    Quote Originally Posted by daimon666 View Post
    you guaranteed resources and virtually zero overselling with xen and kvm as it only allows resources to be allocated once with openvz the same resources can be offered to numerous vps's
    You can oversell memory in Xen and KVM with ballooning. You can oversell CPU and I/O by packing too many VEs on an insufficiently powerful node.

    The important difference is in the approach to virtualization. OpenVZ runs VEs as containers while Xen/KVM run VEs as virtual environments.

    If you look at the big picture -- with OpenVZ, VEs use the host node's kernel and XEN/KVM offer the possibility of using a different kernel.

    While it can be said that it is "easier" to oversell OpenVZ, you can certainly "guarantee" most resources through sane configuration. Overselling is not a feature or consequence, its a management decision.
    Last edited by Microlinux; 10-09-2011 at 09:47 AM.

  4. #4
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    KVM and Xen HVM fully virtualize the host node's hardware, they more or less trick VEs into thinking they are running on a physical server.

    With Xen PV, hardware is not truly virtualized. The benefit is higher performance, but the guest kernel must be specially modified to operate in this environment.

  5. #5
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    Thank you.

    What about reability ? If it is easy to oversell using OpenVZ then I assume the OpenVZ node could be crashed by another node hosted on the same server. Are Xen and KVM more reable than OpenVZ ? Is there some important difference between KVM and Xen ?

  6. #6
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    hello,

    any misconfigured host can be crashed by a guest running wild, but it is "easier" in an openvz environment yes. Nevertheless, a reliable and reasonable ISP will typically prevent such circumstances by proper configuration.

    The main advantages of Xen HVM and KVM are their virtualisation level (full virtualisation, own kernel/OS) and proper separation of containers. Xen is widely being used for full virtualisation, but KVM, especially with its very stable integration into RedHat 6, is definitely on the rise and probably faces a brighter future than Xen. As a customer, unless you already have very, very specific preferences and needs that favour one over the other, could go for either Xen or KVM if you want to have full virtualisation.

    A lot of providers will make a price difference between openVZ and Xen/KVM, which could indicate the level of oversubscription accepted by a provider - but it is by no means proof of that, and it does not mean that said ISP is irresponsible.

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  7. #7
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    Xen PV (Paravirtualization) and Xen HVM (Full-Virtualization, Hardware virtual Machine) are very similar and do the same thing in most respects. The major difference is that Xen HVM utilizes the hardware of the physical server to use the CPU's built-in Intel VT technology to create the virtualization. Xen PV instead uses a slightly modified kernel in the VPS to talk to the hardware of the physical server to accomplish the virtualization. Xen HVM supports non-Linux based operating systems and allows you to install customized operating systems using a VNC Console KVM system and an ISO disc.

    KVM is similar to Xen HVM in the above descriptions.

    Xen Paravirtualization is great if you plan to run any sort of Linux system because you can run different kernels, modules, etc for any unix or linux based system depending on how your provider has things setup - but if you want more than linux, or the ability to swap between windows, and linux - you need either Xen HVM or KVM.


    OpenVZ is more or less a software based emulator creating virtual environments through kernel extensions, but you can't do any sort of customizations (can't installed kernel level modules, can't intsall your own kernel, can't even do device level devices).


    As for over subscription comments - the above are all true, but in general OpenVZ is more often the choice by providers who over subscribe nodes. To simply avoid a troublesome vps hosting company I'd suggest

    A) check reviews

    B) stick to established brands (history, repurtiation, reviews, and real companies rather than resellers/1-man shops)

    C) I'd favor a Xen based solution over an OpenVZ based on. KVM vs Xen, i'd probably pick Xen only because its more established lately, KVM may some day replace Xen as the standard since Centos Kernels now include it. But for 'today' the Xen is the choice I'd look at.
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  8. #8
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    All three have been well described, but long story short; Stick to Xen & KVM for mission critical sites, apps, and game servers. OpenVZ is not very stable due to the amount of overselling guaranteed with most providers.

  9. #9
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    Quote Originally Posted by NickBudi View Post
    Stick to Xen & KVM for mission critical sites, apps, and game servers. OpenVZ is not very stable due to the amount of overselling guaranteed with most providers.
    Its disingenuous to say this.

    OpenVZ as a technology is stable. The individual service a provider sells using this technology may not be. Choosing Xen/KVM in no way means that the service is suddenly more stable and "better" for mission critical applications. They are no different from OpenVZ in that poor management results in poor performance.

    There is little reason to lock yourself into one technology or the other, unless it offers some feature you need that the other doesn't. If you're running a mission critical application, its likely the quality of service and support from the provider is more important than the virtualization technology they use. I don't know many people that are going to run a mission critical application on a $5 VPS, whether its OpenvZ or Xen/KVM.

  10. #10
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    Quote Originally Posted by [CTI] Todd View Post
    Its disingenuous to say this.

    OpenVZ as a technology is stable. The individual service a provider sells using this technology may not be. Choosing Xen/KVM in no way means that the service is suddenly more stable and "better" for mission critical applications. They are no different from OpenVZ in that poor management results in poor performance.

    There is little reason to lock yourself into one technology or the other, unless it offers some feature you need that the other doesn't. If you're running a mission critical application, its likely the quality of service and support from the provider is more important than the virtualization technology they use. I don't know many people that are going to run a mission critical application on a $5 VPS, whether its OpenvZ or Xen/KVM.
    Yes you are correct, OpenVZ itself is stable, it is the way the technology is handled that makes most OpenVZ servers unstable. Is it disingenuous to say that almost all VPS providers that are well known for their quality of service and support use Xen and KVM as opposed to OpenVZ for good reason?

  11. #11
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    You can't really vouche for what is better or more stable. You just have to use whichever fits your model better. If you're going to go with a $4.95 model, there's most likely a reason that it's $.495 for 2GB or whatever the specs are. It's because the server is most likely getting packed to the nines or the server is of lower caliber. Ask a ton of questions about hardware specs, ask for mrtg graphs, anything they will give you to help spec it out. In my opinion, KVM is top, then xen /vmware, then openVZ. That's just my 2c.
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  12. #12
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    Quote Originally Posted by NickBudi View Post
    Is it disingenuous to say that almost all VPS providers that are well known for their quality of service and support use Xen and KVM as opposed to OpenVZ for good reason?
    You're making a straw man argument.

    If there are 20 "good" Xen providers and 10 "good" OpenVZ providers, does that mean I shouldn't consider the OpenVZ providers because there are less of them -- despite the fact the levels of service are equal?

    What "well known" quality providers are you talking about when you say "almost all"? I'm guessing I could come up with a similarly-sized list of providers using OpenVZ/Virtuozzo.

    And let me make it clear, I'm advocating choice based on what you need, not the name of the technology. Xen/KVM/OpenVZ . . . doesn't matter to me as long as its a quality service and supports the featues I need.

  13. #13
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    Quote Originally Posted by Hivelocity Q&A View Post
    In my opinion, KVM is top, then xen /vmware, then openVZ. That's just my 2c.
    Thats also my 2c

    Quote Originally Posted by [CTI] Todd View Post
    You're making a straw man argument.

    If there are 20 "good" Xen providers and 10 "good" OpenVZ providers, does that mean I shouldn't consider the OpenVZ providers because there are less of them -- despite the fact the levels of service are equal?

    What "well known" quality providers are you talking about when you say "almost all"? I'm guessing I could come up with a similarly-sized list of providers using OpenVZ/Virtuozzo.

    And let me make it clear, I'm advocating choice based on what you need, not the name of the technology. Xen/KVM/OpenVZ . . . doesn't matter to me as long as its a quality service and supports the featues I need.
    I didn't say to not consider OpenVZ, just that my personal experience, and in my opinion - the general experience of many users on WHT, is that with Xen and KVM, not only comes very good performance, but great support and quality. By saying "well known", some examples could be Linode or 6Sync. There are certainly many good and well known OpenVZ providers that match the quality and service of those providers, such as Jaguarpc.

  14. #14
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    Quote Originally Posted by NickBudi View Post
    Yes you are correct, OpenVZ itself is stable, it is the way the technology is handled that makes most OpenVZ servers unstable. Is it disingenuous to say that almost all VPS providers that are well known for their quality of service and support use Xen and KVM as opposed to OpenVZ for good reason?
    That's actually a pretty good way to look at it. Premium hosts don't use OpenVZ. I'd understand if they use Virtuozzo with SLM however.
    I've yet to see any "Linode level" hosts with OpenVZ.


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  15. #15
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    Quote Originally Posted by Scientist View Post
    That's actually a pretty good way to look at it. Premium hosts don't use OpenVZ. I'd understand if they use Virtuozzo with SLM however.
    I've yet to see any "Linode level" hosts with OpenVZ.
    I personally have never used JaguarPC but from the positive reviews I've seen here on WHT, they are a fair contender to Linode, and use OpenVZ!

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