CentOS - May 2012 - How to prevent virtual machines running twice on the disk images?

I use KVM on two identical centos5 hosts.

I can live migrate the virtual machines from one to the other and it
works great. Once I do this, I can see VM definitions on both hosts
using virt-manager or virsh list --all
On one machine the VM is running, on the other it reports "shut off".
The disk images are accessible to both host machines and I want to have
only one running a the time (of course). If the VM locks up, I could by
mistake think that the machine is not running and try to start it on the
wrong host.

My question is, how can I prevent host A from starting a "shut off" VM
that actually has been migrated to host B? The VM could actually be
running on any another host. It could also have been crashed. The most
simple solution would be some sort of lock file placed next to the disk
image location, so seen by all hosts. But perhaps there is another way
of working with virt-manager that I am not aware of?

Theo

> -----Original Message-----
> From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On
> Behalf Of Theo Band
> Sent: 11 May 2012 11:51
> To: CentOS mailing list
> Subject: [CentOS] How to prevent virtual machines running twice on the disk
> images?
>
> I use KVM on two identical centos5 hosts.
>
> I can live migrate the virtual machines from one to the other and it works
> great. Once I do this, I can see VM definitions on both hosts using virt-
> manager or virsh list --all On one machine the VM is running, on the other
it
> reports "shut off".
> The disk images are accessible to both host machines and I want to have
only
> one running a the time (of course). If the VM locks up, I could by mistake
> think that the machine is not running and try to start it on the wrong
host.
>
> My question is, how can I prevent host A from starting a "shut
off" VM that
> actually has been migrated to host B? The VM could actually be running on
> any another host. It could also have been crashed. The most simple solution
> would be some sort of lock file placed next to the disk image location, so
> seen by all hosts. But perhaps there is another way of working with virt-
> manager that I am not aware of?
>
> Theo
Are those machines clustered? I used drbd/pacemaker/corosync to achieve
something similar across two CentOS hosts with KVM machines, and the VM can only
be started on the master node where the DRBD drive is mounted and accessible.
Live migrations are fairly easy too with this method

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On Fri, 2012-05-11 at 12:51 +0200, Theo Band wrote:
> I use KVM on two identical centos5 hosts.
> 
> I can live migrate the virtual machines from one to the other and it
> works great. Once I do this, I can see VM definitions on both hosts
> using virt-manager or virsh list --all
> On one machine the VM is running, on the other it reports "shut
off".
> The disk images are accessible to both host machines and I want to have
> only one running a the time (of course). If the VM locks up, I could by
> mistake think that the machine is not running and try to start it on the
> wrong host.
> 
> My question is, how can I prevent host A from starting a "shut
off" VM
> that actually has been migrated to host B? The VM could actually be
> running on any another host. It could also have been crashed. The most
> simple solution would be some sort of lock file placed next to the disk
> image location, so seen by all hosts. But perhaps there is another way
> of working with virt-manager that I am not aware of?
> 
> Theo
This may not be what you are looking for, but you could always dump the
xml of the domain to a file (so you could define/start it again if
needed), then undefine the domain using virsh.

On Fri, 11 May 2012, Theo Band wrote:
> I use KVM on two identical centos5 hosts. [....]
>
> My question is, how can I prevent host A from starting a "shut
off"
> VM that actually has been migrated to host B? The VM could actually 
> be running on any another host. It could also have been crashed. The 
> most simple solution would be some sort of lock file placed next to 
> the disk image location, so seen by all hosts. But perhaps there is 
> another way of working with virt-manager that I am not aware of?
My way of dealing with that is to undefine the domain on host A after 
it's been moved to host B, e.g.,

   virsh migrate --live myvm remote://host-b
   virsh undefine myvm

The CentOS 6 version of virsh allows those operations to be combined:

   virsh migrate --live --persistent --undefinesource myvm ...

-- 
Paul Heinlein
heinlein at madboa.com
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