Xen users - Jul 2013 - Tap2 qcow2 images with libvirt or xm

Hey all,

I''m having trouble using blktap2 qcow2 with Xen. I currently have a Xen
4.2.2 built in a RHEL6 environment with a 3.4 Dom0 kernel from Centos,
which is supposed to have the blktap2 support with qcow2 (and does, sort
of), along with other nice goodies of a new kernel. Libvirt version
0.10.2.4 is also being used, which is specifically built for Xen.

First, the good news - qcow2 images work when launching a VM using xl.
If I stop the xend service and boot the VM using the xl command with a
tap2:qcow2 disk, it will boot without any issues. The problem is if I
try to use the exact same hvm config file with xm (xend running of
course), it will fail with: Error: (''create'',
''-aqcow2:/home/Centos_63_nvidia.qcow2'') failed (512  ) The
config disk
spec is below and the full traceback error in the xend.log is given at
the end of the email.

disk = [ ''tap2:qcow2:/home/Centos_63_nvidia.qcow2,xvda,w'' ]

While I''m fine using xl command line versus xm, the real problem I have
is the same situation arises with using libvirt, which is a necessity
for my work. Using the libvirt xml for the disk given below, I get the
exact same error as when using xend/xm command.

<disk type=''file'' device=''disk''>
      <driver name=''tap'' type=''qcow2''
/>
      <source file=''/home/Centos_63_nvidia.qcow2''/>
      <target dev=''xvda'' bus=''xen''/>
</disk>

I was told via IRC chats that there is a way to enable qcow2 images with
Libvirt and (tangentially) xm, but I have yet to find a solution. Does
anybody have any help or advice for enabling qcow2 Xen images in libvirt?

Thanks,

Andrew




[2013-07-10 14:49:31 7634] ERROR (XendDomainInfo:108) Domain
construction failed
Traceback (most recent call last):
  File
"/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py",
line 106, in create
    vm.start()
  File
"/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py",
line 474, in start
    XendTask.log_progress(31, 60, self._initDomain)
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendTask.py", line
209, in log_progress
    retval = func(*args, **kwds)
  File
"/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py",
line 2920, in _initDomain
    self._createDevices()
  File
"/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py",
line 2365, in _createDevices
    devid = self._createDevice(devclass, config)
  File
"/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py",
line 2327, in _createDevice
    return self.getDeviceController(deviceClass).createDevice(devConfig)
  File
"/usr/lib64/python2.6/site-packages/xen/xend/server/BlktapController.py",
line
174, in createDevice
    device = TapdiskController.create(params, file)
  File
"/usr/lib64/python2.6/site-packages/xen/xend/server/BlktapController.py",
line
286, in create
    return TapdiskController.exc(''create'',
''-a%s:%s'' % (dtype, image))
  File
"/usr/lib64/python2.6/site-packages/xen/xend/server/BlktapController.py",
line
233, in exc
    (args, rc, out, err))
TapdiskException: (''create'',
''-aqcow2:/home/Centos_63_nvidia.qcow2'')
failed (512  )
[2013-07-10 14:49:31 7634] ERROR (xmlrpclib2:181) Internal error
handling xend.domain.create
Traceback (most recent call last):
  File "/usr/lib64/python2.6/site-packages/xen/util/xmlrpclib2.py",
line
134, in _marshaled_dispatch
    response = self._dispatch(method, params)
  File "/usr/lib64/python2.6/SimpleXMLRPCServer.py", line 418, in
_dispatch
    return func(*params)
  File
"/usr/lib64/python2.6/site-packages/xen/xend/server/XMLRPCServer.py",
line 80, in domain_create
    info = XendDomain.instance().domain_create(config)
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendDomain.py",
line
1001, in domain_create
    dominfo = XendDomainInfo.create(config)
  File
"/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py",
line 106, in create
    vm.start()
  File
"/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py",
line 474, in start
    XendTask.log_progress(31, 60, self._initDomain)
  File "/usr/lib64/python2.6/site-packages/xen/xend/XendTask.py", line
209, in log_progress
    retval = func(*args, **kwds)
  File
"/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py",
line 2920, in _initDomain
    self._createDevices()
  File
"/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py",
line 2365, in _createDevices
    devid = self._createDevice(devclass, config)
  File
"/usr/lib64/python2.6/site-packages/xen/xend/XendDomainInfo.py",
line 2327, in _createDevice
    return self.getDeviceController(deviceClass).createDevice(devConfig)
  File
"/usr/lib64/python2.6/site-packages/xen/xend/server/BlktapController.py",
line
174, in createDevice
    device = TapdiskController.create(params, file)
  File
"/usr/lib64/python2.6/site-packages/xen/xend/server/BlktapController.py",
line
286, in create
    return TapdiskController.exc(''create'',
''-a%s:%s'' % (dtype, image))
  File
"/usr/lib64/python2.6/site-packages/xen/xend/server/BlktapController.py",
line
233, in exc
    (args, rc, out, err))
TapdiskException: (''create'',
''-aqcow2:/home/Centos_63_nvidia.qcow2'')
failed (512  )





-- 
Andrew J. Younge
Information Sciences Institute
University of Southern California

> TapdiskException: (''create'',
''-aqcow2:/home/Centos_63_nvidia.qcow2'')
> failed (512  )
Does this end up being passed as arguments to the tap-ctl utility? If it is, it
seems wrong; there should be a space right after the "-a" switch.

I actually figured it out, or at least a around-about solution. What I
did was stop the xend service and then restart libvirtd, which will
apparently trigger libvirt to use the libxl channel instead of the xend
by default.  xl, as confirmed in my previous email, works out of the box
with qcow2 images so using libvirt in this same function also produced
images the desired results.

So, my fix is basically to shutdown xend, restart libvirt to use libxl,
and not worry about using xend/xm again.

I''m assuming this represents some sort of bug or missing functionality
with xend. If there''s a proper place to file a bug for qcow2 support
with xend/xm, please let me know. If xend support for tap2:qcow2 is
truly deprecated then that''s fine by me, I just recommend documenting
this somewhere. Personally I''m just going to stay on with using the
libxl setup, as it currently meets my needs right now.

Thanks,

Andrew

On 7/11/13 4:33 AM, Thanos Makatos wrote:
>> TapdiskException: (''create'',
''-aqcow2:/home/Centos_63_nvidia.qcow2'')
>> failed (512  )
> 
> Does this end up being passed as arguments to the tap-ctl utility? If it
is, it seems wrong; there should be a space right after the "-a"
switch.
> 
-- 
Andrew J. Younge
Information Sciences Institute
University of Southern California

On Thu, 2013-07-11 at 13:36 -0400, Andrew J Younge
wrote:
> I actually figured it out, or at least a around-about solution. What I
> did was stop the xend service and then restart libvirtd, which will
> apparently trigger libvirt to use the libxl channel instead of the xend
> by default.  xl, as confirmed in my previous email, works out of the box
> with qcow2 images so using libvirt in this same function also produced
> images the desired results.
> 
> So, my fix is basically to shutdown xend, restart libvirt to use libxl,
> and not worry about using xend/xm again.
BTW I''d recommend disabling starting xend at boot rather than shutting
it down.
> I''m assuming this represents some sort of bug or missing
functionality
> with xend. If there''s a proper place to file a bug for qcow2
support
> with xend/xm, please let me know. If xend support for tap2:qcow2 is
> truly deprecated then that''s fine by me, I just recommend
documenting
> this somewhere.
xend itself is deprecated. It''s mentioned in the wiki e..g
http://wiki.xen.org/wiki/Xen_Release_Features and
http://wiki.xen.org/wiki/XEND as well as the xm manpage.
>  Personally I''m just going to stay on with using the
> libxl setup, as it currently meets my needs right now.
This is the right thing to be using as well.

Ian.