Xen users - May 2013 - blktap2 and qcow2 images

Hi There,

I''ve been trying to get this to work for the last couple of days, but
found
no information on the internet that would help. Essentially, a tap-ctl
opencommand with a qcow2 image does not work (error code 2) and
produces the
following line in syslog:

tap-ctl: tap-err:tap_ctl_open: open failed, err -2

I have attached a shell script that reproduces the problem on my machine.
The same script works fine if I use image format raw and the aio: prefex to
the -a option.

My machine is basically a clean Ubuntu 12.04 desktop 64bit machine with
everything up to date.

Cheers,
Seb



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> -----Original Message-----
> From: xen-users-bounces@lists.xen.org [mailto:xen-users-
> bounces@lists.xen.org] On Behalf Of Sebastian Unger
> Sent: 08 May 2013 23:03
> To: xen-users@lists.xen.org
> Subject: [Xen-users] blktap2 and qcow2 images
> 
> Hi There,
> 
> I''ve been trying to get this to work for the last couple of days,
but
> found no information on the internet that would help. Essentially, a
> tap-ctl open command with a qcow2 image does not work (error code 2)
> and produces the following line in syslog:
> 
> tap-ctl: tap-err:tap_ctl_open: open failed, err -2
> 
> I have attached a shell script that reproduces the problem on my
> machine. The same script works fine if I use image format raw and the
> aio: prefex to the -a option.
> My machine is basically a clean Ubuntu 12.04 desktop 64bit machine with
> everything up to date.
> Cheers,
> Seb
AFAIK blktap2.5 does not support qcow2. I grep''ed in
xen-unstable.hg/tools/blktap2 for "qcow2" and didn''t find
anything, so I believe blktap2 doesn''t support qcow2 either. The
"-2" error you''re getting stands for ENOENT, so I think is
tapdisk complaining it can''t find the driver for qcow2 (not the image
file). The examples in http://wiki.xen.org/wiki/Blktap2 insinuate that qcow2 is
supported, anybody knows anything about that?

Hi Thanos,

thanks for responding. I don''t have access to the code. Could you by
any
chance quote a list of supported formats here.

Cheers,
Seb
On 10/05/2013 2:49 AM, "Thanos Makatos"
<thanos.makatos@citrix.com> wrote:
> > -----Original Message-----
> > From: xen-users-bounces@lists.xen.org [mailto:xen-users-
> > bounces@lists.xen.org] On Behalf Of Sebastian Unger
> > Sent: 08 May 2013 23:03
> > To: xen-users@lists.xen.org
> > Subject: [Xen-users] blktap2 and qcow2 images
> >
> > Hi There,
> >
> > I''ve been trying to get this to work for the last couple of
days, but
> > found no information on the internet that would help. Essentially, a
> > tap-ctl open command with a qcow2 image does not work (error code 2)
> > and produces the following line in syslog:
> >
> > tap-ctl: tap-err:tap_ctl_open: open failed, err -2
> >
> > I have attached a shell script that reproduces the problem on my
> > machine. The same script works fine if I use image format raw and the
> > aio: prefex to the -a option.
> > My machine is basically a clean Ubuntu 12.04 desktop 64bit machine
with
> > everything up to date.
> > Cheers,
> > Seb
>
> AFAIK blktap2.5 does not support qcow2. I grep''ed in
> xen-unstable.hg/tools/blktap2 for "qcow2" and didn''t
find anything, so I
> believe blktap2 doesn''t support qcow2 either. The "-2"
error you''re getting
> stands for ENOENT, so I think is tapdisk complaining it can''t find
the
> driver for qcow2 (not the image file). The examples in
> http://wiki.xen.org/wiki/Blktap2 insinuate that qcow2 is supported,
> anybody knows anything about that?
>

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> Hi Thanos,
> thanks for responding. I don''t have access to the code. Could you
by
> any chance quote a list of supported formats here.
> Cheers,
> Seb
I''ve only used RAW and VHD so far.
tools/blktap2/drivers/tapdisk-disktype.c says that qcow, ram, block_cache,
vhd_index, log, and remus are also supported. Apart from qcow, I don''t
know about the rest.

Mhmm, I''ve run a quick test, but it doesn''t work with qcow
either. Although
the error is slightly different:

tap-ctl: tap-err:tap_ctl_open: open failed, err -38

Do you know what that error (-38) means?

Out of curiosity, does qcow support in-file snapshots like qcow2?

Cheers,
Seb
On 14/05/2013 8:48 PM, "Thanos Makatos"
<thanos.makatos@citrix.com> wrote:
> > Hi Thanos,
> > thanks for responding. I don''t have access to the code. Could
you by
> > any chance quote a list of supported formats here.
> > Cheers,
> > Seb
>
> I''ve only used RAW and VHD so far.
> tools/blktap2/drivers/tapdisk-disktype.c says that qcow, ram, block_cache,
> vhd_index, log, and remus are also supported. Apart from qcow, I
don''t know
> about the rest.
>

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> Mhmm, I''ve run a quick test, but it doesn''t work with
qcow either.
> Although the error is slightly different:
> tap-ctl: tap-err:tap_ctl_open: open failed, err -38 Do you know what
> that error (-38) means?
38 stands for ENOSYS (you can consult /usr/include/asm-generic/errno-base.h and
/usr/include/asm-generic/errno.h for translating error codes). It means that
something is not implemented, maybe support for qcow..
> Out of curiosity, does qcow support in-file snapshots like qcow2?
No idea.

So basically xen only supports physical devices and plain image files as
disk back- ends?! To use snapshots one has to use LVM?

That seems pretty thin for something that I thought was a production
quality virtualization solution.

A well, I guess I''ll go back to virtualbox then.

Thanks for your help Thanos.

Cheers,
Seb
On 14/05/2013 9:06 PM, "Thanos Makatos"
<thanos.makatos@citrix.com> wrote:
> > Mhmm, I''ve run a quick test, but it doesn''t work
with qcow either.
> > Although the error is slightly different:
> > tap-ctl: tap-err:tap_ctl_open: open failed, err -38 Do you know what
> > that error (-38) means?
>
> 38 stands for ENOSYS (you can consult
> /usr/include/asm-generic/errno-base.h and /usr/include/asm-generic/errno.h
> for translating error codes). It means that something is not implemented,
> maybe support for qcow..
>
> > Out of curiosity, does qcow support in-file snapshots like qcow2?
>
> No idea.
>

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> So basically xen only supports physical devices and plain image files
> as disk back- ends?! To use snapshots one has to use LVM?
If you want snapshots why don''t you use the VHD format? You
don''t necessarily have to use LVM for this; VHD images can be stored
into ordinary files.

Hi Thanos,

according to the man page and my experiments VHD does not support snapshots:

$ qemu-img snapshot -c base disk1.vhd
qemu-img: Could not create snapshot ''base'': -95 (Operation not
supported)

$ qemu-img create -f vpc -o backing_file=disk1.vhd,backing_fmt=vpc
disk1-snap1.vhd
Unknown option ''backing_file''
qemu-img: Invalid options for file format ''vpc''.

So has anybody out there gotten file image based snapshots working with
xen? Because from everything I''m seeing it seems to be simply not
supported, but that just seems to be such a basic bit of functionality,
that I find that hard to believe.

Cheers,
Seb



On Tue, May 14, 2013 at 10:13 PM, Thanos Makatos
<thanos.makatos@citrix.com>wrote:
> > So basically xen only supports physical devices and plain image files
> > as disk back- ends?! To use snapshots one has to use LVM?
>
> If you want snapshots why don''t you use the VHD format? You
don''t
> necessarily have to use LVM for this; VHD images can be stored into
> ordinary files.
>

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Xen-users@lists.xen.org
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> Hi Thanos,
> according to the man page and my experiments VHD does not support
> snapshots:
> 
> $ qemu-img snapshot -c base disk1.vhd
> qemu-img: Could not create snapshot ''base'': -95
(Operation not
> supported)
> 
> $ qemu-img create -f vpc -o backing_file=disk1.vhd,backing_fmt=vpc
> disk1-snap1.vhd Unknown option ''backing_file''
> qemu-img: Invalid options for file format ''vpc''.
> So has anybody out there gotten file image based snapshots working with
> xen? Because from everything I''m seeing it seems to be simply not
> supported, but that just seems to be such a basic bit of functionality,
> that I find that hard to believe.
I don''t think this is the right way to perform snapshots on VHD files,
use vhd-util instead:
vhd-util snapshot -p original-file-image.vhd -n snapshot-file-image.vhd