[Qemu-devel] Citrix VHD implementation

[Qemu-devel] Citrix VHD implementation

[Philipp Schmid]

Hi,

as some of you might know, citrix has open sourced the library they  
use to read and write to VHD disk images.

I'm trying to implementing a block-vhd driver on top of libvhd, but my  
C knowledge is very limited as I haven't written any code in C for a  
long time.

What I already have is a git repo with the libvhd code extracted from  
the xen-unstable tree with the blocktap2 patches applied.
libvhd also comes with vhd-util, a utility to create and modify vhd  
images.
I have only tried to compile it on ubuntu 8.10, so it might not work  
on your system.

The git repo is hosted on github: http://github.com/schmidp/libvhd/tree/master

Are there any docs about how to implement a block driver for qemu?
As far as I have seen from the qemu code, block_int.h defines the  
interface and you register your driver in block.c, but that is all  
I've found out so far.

I will try to implement a synchrones driver first as it seems more  
simple...


greets Philipp

ps: I'm not affiliated with citrix in any way

Re: [Qemu-devel] Citrix VHD implementation

[Stefano Stabellini]

Philipp Schmid wrote:

> Hi,
> 
> as some of you might know, citrix has open sourced the library they  
> use to read and write to VHD disk images.
> 
> I'm trying to implementing a block-vhd driver on top of libvhd, but my  
> C knowledge is very limited as I haven't written any code in C for a  
> long time.
> 
> What I already have is a git repo with the libvhd code extracted from  
> the xen-unstable tree with the blocktap2 patches applied.
> libvhd also comes with vhd-util, a utility to create and modify vhd  
> images.
> I have only tried to compile it on ubuntu 8.10, so it might not work  
> on your system.
> 
> The git repo is hosted on github: http://github.com/schmidp/libvhd/tree/master
> 
> Are there any docs about how to implement a block driver for qemu?
> As far as I have seen from the qemu code, block_int.h defines the  
> interface and you register your driver in block.c, but that is all  
> I've found out so far.
> 
> I will try to implement a synchrones driver first as it seems more  
> simple...

Why don't you just use blktap2 to create a raw device from the vhd file
and then open the raw device in qemu?

Re: [Qemu-devel] Citrix VHD implementation

[Daniel P. Berrange]

On Mon, Mar 02, 2009 at 11:41:25AM +0000, Stefano Stabellini wrote:
> Philipp Schmid wrote:
> 
> > Hi,
> > 
> > as some of you might know, citrix has open sourced the library they  
> > use to read and write to VHD disk images.
> > 
> > I'm trying to implementing a block-vhd driver on top of libvhd, but my  
> > C knowledge is very limited as I haven't written any code in C for a  
> > long time.
> > 
> > What I already have is a git repo with the libvhd code extracted from  
> > the xen-unstable tree with the blocktap2 patches applied.
> > libvhd also comes with vhd-util, a utility to create and modify vhd  
> > images.
> > I have only tried to compile it on ubuntu 8.10, so it might not work  
> > on your system.
> > 
> > The git repo is hosted on github: http://github.com/schmidp/libvhd/tree/master
> > 
> > Are there any docs about how to implement a block driver for qemu?
> > As far as I have seen from the qemu code, block_int.h defines the  
> > interface and you register your driver in block.c, but that is all  
> > I've found out so far.
> > 
> > I will try to implement a synchrones driver first as it seems more  
> > simple...
> 
> Why don't you just use blktap2 to create a raw device from the vhd file
> and then open the raw device in qemu?

There is no blktap unless you have Xen userspace and the Xen kernel
installed. Having a native vhd QEMU driver would avoid the need to
install Xen just to use VHD format images under non-Xen QEMU/KVM.

Daniel
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Re: [Qemu-devel] Citrix VHD implementation

[Anthony Liguori]

Philipp Schmid wrote:
> Hi,
>
> as some of you might know, citrix has open sourced the library they 
> use to read and write to VHD disk images.
>
> I'm trying to implementing a block-vhd driver on top of libvhd, but my 
> C knowledge is very limited as I haven't written any code in C for a 
> long time.
>
> What I already have is a git repo with the libvhd code extracted from 
> the xen-unstable tree with the blocktap2 patches applied.
> libvhd also comes with vhd-util, a utility to create and modify vhd 
> images.
> I have only tried to compile it on ubuntu 8.10, so it might not work 
> on your system.
>
> The git repo is hosted on github: 
> http://github.com/schmidp/libvhd/tree/master

I took a look at the libvhd code.  I don't think I'd want to replace the 
QEMU vpc code with it so if you plan on pushing your work upstream 
eventually, you may want to consider other options.

The libvhd code provides a synchronous IO interface.  While it's 
relatively clean wrt using a global context, you'd have to hold a lock 
to ensure thread safety if you were to attempt to make it asynchronous 
with threads.  Because the read function blocks (while you were holding 
the global context lock), it's more or less impossible to use threads to 
make it asynchronous while supporting multiple requests.

The sum of this means that it's not going to fit into QEMU very well and 
doesn't make sense as a long term mechanism for VHD support in QEMU.

Ideally, the libvhd code would be implemented entirely with asynchronous 
IO functions that could be pluggable.  That's quite a different beast 
though from the current implementation.

Regards,

Anthony Liguori

> Are there any docs about how to implement a block driver for qemu?
> As far as I have seen from the qemu code, block_int.h defines the 
> interface and you register your driver in block.c, but that is all 
> I've found out so far.
>
> I will try to implement a synchrones driver first as it seems more 
> simple...
>
>
> greets Philipp
>
> ps: I'm not affiliated with citrix in any way
>
>
>

Re: [Qemu-devel] Citrix VHD implementation

[Kevin Wolf]

Hi Philipp,

Philipp Schmid schrieb:
> as some of you might know, citrix has open sourced the library they use
> to read and write to VHD disk images.
>
> I'm trying to implementing a block-vhd driver on top of libvhd, but my C
> knowledge is very limited as I haven't written any code in C for a long
> time.

qemu already has a basic driver for VHD images. It's called block-vpc  
and currently works with dynamic disks. If you want to improve VHD  
support in qemu, I think you should start there. It's completely  
synchronous (changing this could be one of the next steps) and small  
enough to be easy to start with.

> Are there any docs about how to implement a block driver for qemu?
> As far as I have seen from the qemu code, block_int.h defines the
> interface and you register your driver in block.c, but that is all I've
> found out so far.

It's more or less all you need. ;-)

Kevin