libvirt users - May 2015 - Re: Proper way to reference a disk

Hi,
We are running on CentOS 7.0. We want to move our VMs (kvm/qemu) from
qcow/image files to logical volumes under LVM.

I have moved one of them but I had a little trouble editing the disk
properly and I wonder what is the correct way to do it. Specifically, in
the XML under the devices/disk section why does the source say "source
file=",
and not "source dev="?  The reason I ask is that I have a number of
other
machines to move and I don't want to mess them up.

The source file entry for the disk was like this in our xml file:
<source file='/var/lib/libvirt/images/pxetest.qcow2'/>

I got the vm to work by converting the image to raw, then dumping via dd
into the LV, and editing the XML using virsh as follows:
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'
cache='none'/>
      <*source file*='/dev/kvm_temp/pxetest'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00'
slot='0x04'
function='0x0'/>
    </disk>

By comparison I have a test machine on another real host upon which I
loaded a VM and put its disk into a logical volume. Its disk section looks
like this:

    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none'
io='native'/>
      <*source dev*='/dev/kvm_hosts/pxetest2'/>
      <target dev='hda' bus='ide'/>
      <address type='drive' controller='0' bus='0'
target='0' unit='0'/>
    </disk>

The source file / source dev thing really has me wondering.


*- Mike Schwager*

*  Linux Network Engineer, Mocho Trading LLC*

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On 05/06/2015 02:05 PM, Michael Schwager wrote:
> Hi,
> We are running on CentOS 7.0. We want to move our VMs (kvm/qemu) from
> qcow/image files to logical volumes under LVM.
> 
> I have moved one of them but I had a little trouble editing the disk
> properly and I wonder what is the correct way to do it. Specifically, in
> the XML under the devices/disk section why does the source say "source
file=",
> and not "source dev="?  The reason I ask is that I have a number
of other
> machines to move and I don't want to mess them up.
> 
> The source file entry for the disk was like this in our xml file:
> <source file='/var/lib/libvirt/images/pxetest.qcow2'/>
> 
> I got the vm to work by converting the image to raw, then dumping via dd
> into the LV, and editing the XML using virsh as follows:
>     <disk type='file' device='disk'>
>       <driver name='qemu' type='raw'
cache='none'/>
>       <*source file*='/dev/kvm_temp/pxetest'/>
The key for what <source> looks like is the <disk
type='...'> attribute
a couple lines earlier.  When type='file', then <source> lists a
file='...' absolute path, and libvirt treats it as a file (which sort of
works, except when querying for how much of the file is actually
allocated, because it assumes that stat() alone is good enough)
> By comparison I have a test machine on another real host upon which I
> loaded a VM and put its disk into a logical volume. Its disk section looks
> like this:
> 
>     <disk type='block' device='disk'>
>       <driver name='qemu' type='raw'
cache='none' io='native'/>
>       <*source dev*='/dev/kvm_hosts/pxetest2'/>
And here, you used type='block', so <source> must list
dev='...' and
give an absolute path to a block device (and here, when libvirt is
computing how much of the device is allocated, it relies on more than
just stat(), which can give more accurate numbers when doing things like
qcow2 format atop an LVM block device).
>       <target dev='hda' bus='ide'/>
>       <address type='drive' controller='0'
bus='0' target='0' unit='0'/>
>     </disk>
> 
> The source file / source dev thing really has me wondering.
Hopefully I answered it.  As a rule of thumb, if it is a block device,
use type='block' (or type='lun' if you need sgio passthrough
support)
and <source dev='...'>); otherwise use type='file' and
<source file='...'.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

On Wed, May 6, 2015 at 4:14 PM, Eric Blake <eblake@redhat.com> wrote:
> > The source file / source dev thing really has me wondering.
> Hopefully I answered it.
>
​Yes you did, thanks.​

​Furthermore,​

> ​​which can give more accurate numbers when doing things like
> qcow2 format atop an LVM block device).
>

​...This has me intrigued. We are running qcow2 format in files, and the
instructions I've seen have you converting qcow2 to raw and then dd into
LVM. But is there an advantage to running qcow2 on LVM? Is there an example
of this in action?

Thanks again.


*- Mike Schwager*

*  Linux Network Engineer, Mocho Trading LLC*
*  312-646-4783 Phone    312-637-0011 Cell    312-957-9804 Fax*

-- 
This message is for the named person(s) use only. It may contain 
confidential proprietary or legally privileged information. No 
confidentiality or privilege is waived or lost by any mistransmission. If 
you receive this message in error, please immediately delete it and all 
copies of it from your system, destroy any hard copies of it and notify the 
sender. You must not, directly or indirectly use, disclose, distribute, 
print, or copy any part of this message if you are not the intended 
recipient. Mocho Trading LLC reserves the right to monitor all e-mail 
communications through its networks. Any views expressed in this message 
are those of the individual sender, except where the message states 
otherwise and the sender is authorized to state them to be the views of any 
such entity.