Xen devel - Mar 2005 - dynamically allocating loopback devices?

My users'' VM disks are all loopback files. Right now they have to run
losetup on each of their files before starting their VMs. This means
we have to statically allocate loopback devices. And if they want to
change the disks used they have to remember to run "losetup -d". Is
there any way, without my writing extra glue logic, for a user to say
in his config file "these are my loopback files, I don''t care what
loopback devices they get bound to, just bind them for me and then
start my VM"?

Thanks.

                             -Kip


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> My users'' VM disks are all loopback files. Right now they have to
run
> losetup on each of their files before starting their VMs. This means
> we have to statically allocate loopback devices. And if they want to
> change the disks used they have to remember to run "losetup -d".
Is
> there any way, without my writing extra glue logic, for a user to say
> in his config file "these are my loopback files, I don''t care
what
> loopback devices they get bound to, just bind them for me and then
> start my VM"?
Check out the ''file:'' syntax for specifying vbd''s.

Ian


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That allows me to bypass the losetup, but what about device
allocation? - the examples I see all look like
disk = [ ''file:/u/kmacy/mdroot-5.3,loop7,w'' ]

What I''d really like is to be able to drop the second parameter, or 
do something like

disk = [ ''file:/u/kmacy/mdroot-5.3,*,w'' ]

-Kip

On Fri, 11 Mar 2005 21:55:39 -0000, Ian Pratt <m+Ian.Pratt@cl.cam.ac.uk>
wrote:
> 
> > My users'' VM disks are all loopback files. Right now they
have to run
> > losetup on each of their files before starting their VMs. This means
> > we have to statically allocate loopback devices. And if they want to
> > change the disks used they have to remember to run "losetup
-d". Is
> > there any way, without my writing extra glue logic, for a user to say
> > in his config file "these are my loopback files, I don''t
care what
> > loopback devices they get bound to, just bind them for me and then
> > start my VM"?
> 
> Check out the ''file:'' syntax for specifying
vbd''s.
> 
> Ian
>

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> That allows me to bypass the losetup, but what about device
> allocation? - the examples I see all look like
> disk = [ ''file:/u/kmacy/mdroot-5.3,loop7,w'' ]
file: does do the loop device allocation. The second parameter is the
device you want the vbd to appear in the guest as, i.e. usually sda1 or
hda1.

Ian


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I was confused by the fact that xm insists on it being present in DOM0.

disk = [ ''file:/u/kmacy/mdroot-5.3,xbd0,w'' ]

kmacy@shemp sh -x ~/xentmp/xenargs.shemp
+ xm create -f /u/kmacy/xentmp/freebsd.shemp -c
name=xen-vm2.lab.netapp.com vmid=10 kernel=/u/kmacy/kernel
Using config file "/u/kmacy/xentmp/freebsd.shemp".
Error: Error creating domain: vbd: Device not found: xbd0

Thanks.

          -Kip

On Fri, 11 Mar 2005 22:09:04 -0000, Ian Pratt <m+Ian.Pratt@cl.cam.ac.uk>
wrote:
> > That allows me to bypass the losetup, but what about device
> > allocation? - the examples I see all look like
> > disk = [ ''file:/u/kmacy/mdroot-5.3,loop7,w'' ]
> 
> file: does do the loop device allocation. The second parameter is the
> device you want the vbd to appear in the guest as, i.e. usually sda1 or
> hda1.
> 
> Ian
>

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> I was confused by the fact that xm insists on it being present in DOM0.
>
> disk = [ ''file:/u/kmacy/mdroot-5.3,xbd0,w'' ]
>
> kmacy@shemp sh -x ~/xentmp/xenargs.shemp
> + xm create -f /u/kmacy/xentmp/freebsd.shemp -c
> name=xen-vm2.lab.netapp.com vmid=10 kernel=/u/kmacy/kernel
> Using config file "/u/kmacy/xentmp/freebsd.shemp".
> Error: Error creating domain: vbd: Device not found: xbd0
That''s because it used to stat the device node in dom0 so that it would
know
what major / minor to tell the domain to use.  Xend has since been patched to 
not require this, I''m not sure what trees it was checked into, though

Cheers,
Mark
> Thanks.
>
>           -Kip
>
> On Fri, 11 Mar 2005 22:09:04 -0000, Ian Pratt
<m+Ian.Pratt@cl.cam.ac.uk>
wrote:
> > > That allows me to bypass the losetup, but what about device
> > > allocation? - the examples I see all look like
> > > disk = [ ''file:/u/kmacy/mdroot-5.3,loop7,w'' ]
> >
> > file: does do the loop device allocation. The second parameter is the
> > device you want the vbd to appear in the guest as, i.e. usually sda1
or
> > hda1.
> >
> > Ian
>
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> Ahh - makes sense. It is, however, a rather linux-centric approach.
Yep!  How do you get Xend to tell FreeBSD what disks to expect?

I imagine this''ll get fixed in the new tools.  It would be rather 
straightforward to extend the code in Xend to support other numbering 
schemes, so maybe it''ll get fixed sooner than that.

Cheers,
Mark


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> That''s because it used to stat the device node in dom0 so that it
would know
> what major / minor to tell the domain to use.  Xend has since been patched
to
> not require this, I''m not sure what trees it was checked into,
though
> 
> Cheers,
> Mark
Ahh - makes sense. It is, however, a rather linux-centric approach.

     -Kip


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> > That''s because it used to stat the device node in dom0 so 
> that it would know
> > what major / minor to tell the domain to use.  Xend has 
> since been patched to
> > not require this, I''m not sure what trees it was checked 
> into, though
> 
> Ahh - makes sense. It is, however, a rather linux-centric approach.
It sucks. The obvious thing to do would be to pass a string, that the
guest then looks up, but I seem to recall that doing this at boot time
was rather ugly on linux.

In the meantime, I''ll take a patch that adds some well known *BSD
devices to the exception table in xend.

Ian


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