[ale] Citrix Xen

Fri Jul 13 16:11:54 EDT 2012

I think KVM has always depended on hardware visualization extensions
provided by the CPU, making it similar to HVM mode Xen.[1]

You can use Qemu without KVM and it will do emulation,[2] but if you
use KVM, then you don't need qemu's emulation---it becomes a sort of
shim for interacting with the hardware-virtualized VM, and it can
provide stuff like virtualized network interfaces.

  1. http://www.linux-kvm.org/page/FAQ#What_do_I_need_to_use_KVM.3F
  2. http://www.linux-kvm.org/page/FAQ#How_can_I_check_that_I.27m_not_falling_back_to_QEMU_with_no_hardware_acceleration.3F

On Fri, Jul 13, 2012 at 1:30 PM, Michael Trausch <mike at trausch.us> wrote:
> Modern Xen is much better. Both Dom0 and DomU are in the mainline kernel,
> and so no custom kernels are needed anymore. Am using it with Xen 4 on
> Debian 6 for about a dozen VMs.
>
> The only problem with the distribution of Xen in Debian is that it depends
> on the external world for a few things, like bootstrapping a new VM. I
> haven't used anything that stacks on top of Xen or the scripts provided by
> Debian, though and I suspect that there is a healthy management system for
> it somewhere on the Internet. I would like to play more with getting Xen to
> be automatically controlled in a setup not unlike Linode's setup, someday,
> with the ability to do automatic provisioning and system setup, as well as
> things like live migration.
>
> I am curious as to how KVM is more flexible than Xen, though. The overhead
> is far greater since it is actually emulating hardware, and the only
> interface to KVM that I am aware of is the KVM fork of QEMU, which is
> essentially just QEMU, as I understand it, with the KVM system being used to
> accelerate the CPU emulation. Or is there now a command line interface that
> allows things to be more efficient yet and drop things like the VGA chipset
> and other hardware emulation?
>
> Anyway, just wanted to really mention the face that the upstream/mainline
> kernel supports execution on Xen natively now. :-) I think that happened
> extremely late in the 2.6 series, maybe 2.6.38 or 2.6.39, IIRC.
>
> --
> Sent from my Android device. Sorry for any typos,
> autocorrect is sometimes a pain in the rear.
>
> On Jul 13, 2012 12:23 PM, "JD" <jdp at algoloma.com> wrote:
>>
>> On 07/13/2012 10:40 AM, Chuck Payne wrote:
>> > Guys,
>> >
>> > Anyone using Citirix Xen? I just started a new job and they are using
>> > it as their Virtual Server, but I am seeing a lot of issues, before I
>> > go and recommend a change, I like to hear some feed back, but Pro and
>> > Con. I am a bit basis as I have use VMWare ESXi and KVM.
>> >
>> > Just so note some of the issues we had so far, lots of kernel panics
>> > with NFS. Virtual Machines being rebuild because of issues with the vm
>> > themselves, such as drives going bad.  Not being able to reboot
>> >
>>
>> I've been using paravirtual Xen since mid-2008, not the Citrix specific
>> version.
>> To get more flexibility, we're migrating to KVM.  We've migrated off ESXi,
>> Xen,
>> and VirtualBox VMs so far. These are servers. For desktop virtualization,
>> both
>> VirtualBox and KVM are used still.  There are still 5 Xen DomUs to be
>> migrated.
>>
>> Our use of Xen was pretty simple. Only Linux clients running the same
>> distro and
>> no HVM. I tried to get a Windows VM running using HVM, but didn't have the
>> luxury of lots of downtime on those hosts, so when it didn't immediately
>> work
>> with a few hours of effort, we stopped trying.  Windows DomUs run well
>> enough
>> under KVM. I've have a Windows7 Media Center recoding TV with dual network
>> tuners that way for about 9 months. HDD performance is a different issue
>> with
>> Windows DomUs, however.
>>
>> Xen was pretty solid, but when the hostOS had a kernel patch, I found it
>> was
>> about 15% likely that it wouldn't boot up with the new kernel. It isn't
>> fun to
>> see a kernel panic at that point, usually around 4am on a Saturday
>> morning.  I'd
>> have to drop back to a previous kernel for about a week as other modules
>> were
>> released to handle the newer kernel. This issue happened about once a
>> year. Once
>> Xen was up and running, it stayed up. Never crashed unless there was a
>> hardware
>> error.  Hardware failures will take any hyper-visor down.
>>
>> I think we run Xen v3.2 .... each version has different capabilities and
>> compatibilities. I'm certain we could run a newer version of Xen, if we
>> wanted
>> to manage kernels.
>>
>> I manually created each Xen VM config file and learned a few lessons about
>> forcing MAC addresses to the client OSes so network monitoring was useful
>> at
>> all. Without that, new, random MACs were created at every DomU boot.
>>
>> Also we only used the CLI interface (xm) to manage each VM.  It appears to
>> me
>> that the virsh CLI interface provides similar capabilities, so the that
>> part of
>> the learning curve isn't steep at all.  I'm pretty certain that newer
>> libvirt
>> and virt-manager handle recently created Xen VMs now - at least for the
>> last
>> year or so. It doesn't work with our old Xen Dom0, however.
>>
>> If they are big into Xen and happy with it, I don't know that I'd change,
>> unless
>> they are unhappy with the support or license costs. It also matters which
>> clientOS is being run the most.  For mostly Linux VMs, then KVM is a good
>> choice, but if mostly Windows DomUs, the choice becomes more difficult.
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