[libvirt-users] migrating from XenServer / XCP to libvirt/KVM

[Daniel P. Berrange]

On Wed, Mar 15, 2017 at 10:10:36AM +0000, Daniel Pocock wrote:
> 
> I've been using XenServer / XCP (with the "xe" toolset) for a number of
> years for hosting servers used for development and free software
> projects and I'm now looking at migrating all those environments to
> libvirt / KVM.
> 
> I had a look at the wiki[1] already and didn't see XenServer mentioned
> there.

Yeah, there was a tiny bit of work done to support the XenServer API,
but the person working on it changed jobs and no one else stepped
forward to continue it.

> 
> Could anybody help comment on or explain a few things:
> 
> - comparison between XenServer and libvirt: I notice many similar
> concepts, for example, XenServer has storage repositories and libvirt
> has storage pools.  However, in many examples such as this[2] it appears
> that libvirt requires more manual effort.  For example, when you create
> a storage repository in XenServer, the tool takes a physical block
> device as input and creates the necessary volume group, logical volume
> and a filesystem.  In that libvirt example, it appears each of those
> steps must be done manually.  The networking examples are similar: the
> XenServer tools do all the Open vSwitch stuff behind the scenes but in
> the libvirt examples it appears necessary to create the bridge manually
> before telling libvirt about it.  Is this all correct or are the
> examples I've seen out of date?  Do more recent (or future) releases of
> libvirt aim to automate/hide more of these things?

NB, comparing XenServer with libvirt is a bit of an apples & oranges
comparison, as they have different goals. So yeah, libvirt requires
more steps in managing the storage, but this also allows you todo
things you can't do with XenServer. Also, note that most administrators
would not use libvirt directly, but rather use it indirectly via some
management app that introduces higher level abstractions (eg, oVirt,
OpenStack, or in simpler cases virt-manager etc). 

> - what is best practice for virtual disk images?  Does libvirt always
> use files (like XenServer) or can/should block devices be used
> directly?  If using files, is any special care needed to avoid block
> alignment problems?

This is a matter for the individual hypervisor beneath libvirt. Each
has its own set of supported virtual disk image backends. QEMU/KVM
can run on prettty much anything, file, block device, network storage,
etc

> - are there any guides or tools recommended for migrating small
> XenServer environments (less than 50 domains on a single physical node)
> to libvirt + KVM?
> 
> - is there a list of small things that need to change in a VM before
> running it under libvirt / KVM?  I'm guessing this might include
> bootloader config, updating the kernel, changing block device names in
> /etc/fstab and changing network device names in scripts - are there any
> others?
> 
> - can anybody comment on more tricky issues that may arise in such a
> migration, for example, will a Windows VM be likely to run without
> modification when migrated from XenServer to libvirt/KVM or will it need
> extra drivers added or anything else before the migration?

Yes, there are lots of things which need changing when moving an existing
guest to a new hypervisor. The best thing is to try to use a pre-existing
tool for conversion. eg virt-v2v

   http://libguestfs.org/virt-v2v.1.html

though I'm not sure on the level of support for XenServer there. You
probably just have to give it a try on a guest and see how it works.
It clones the disk image, so your original guest won't be broken.

Regards,
Daniel
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