Open Stack

On Fri, Jul 06, 2012 at 02:35:14AM -0700, Eric W. Biederman wrote:
> "Daniel P. Berrange" <berrange at redhat.com> writes:
> > The part of the problem is that I have a /dev/sdf, or even a
> > /dev/volgroup00/logvol3 in the host (with whatever major:minor
> > number that implies), and I want to be able to make it always
> > appear as /dev/sda  in the container (with the correspondingly
> > different major:minor number).  I'm guessing this is what Serge
> > was refering to as the 'device' namespace problem
> 
> Getting the device to always appear with the name /dev/sda is easy.
> 
> Where does the need to have a specific device come from?  I would have
> thought by now that hotplug had been around long enough that in general
> user space would not care.
> 
> The only case that I know of where keeping the same device number seems
> reasonable is in the case of live migration an application, in order to
> avoid issues with stat changing for the same file over the transition,
> and I think a synthesized hotplug event could probably handle that case.
> 
> Is there another case besides buggy applications that have hard
> coded device numbers that need specific device numbers?

There isn't any particular buggy application we're trying to avoid
here. We're just trying to provide an piece of OpenStack functionality
to LXC in the same way as its provided to KVM.

With a basic OpenStack instance, you just get the root filesystem
from the image you booted, whose contents are transient (ie thrown
away on shutdown). It is possible to tell OpenStack to attach one
or more block devices to a running instance, which give you some
persistent storage.

The end user API for this lets the host admin specify the device
name that the block device will appear as inside the instance.

eg, with KVM you'd invoke:

 # nova volume-attach myguest  mystoragevol1 /dev/vdb
 # nova volume-attach myguest  mystoragevol2 /dev/vdc

Obviously with KVM this just works, because you have a level of
indirection between host & guest device names via virtio-blk.

The desire is to be able to wire up LXC in a similar way

 # nova volume-attach myguest  mystoragevol1 /dev/sdb
 # nova volume-attach myguest  mystoragevol2 /dev/sdc

So it is really the host admin specifying that they want to provide
the container with a '/dev/sdb' device, regardless of what the actual
device node on the host is (it could be an iSCSI LUN, multipath LUN,
LVM volume, or whatever). So I'm really looking to have the container
visible device name be independent of host name.

Regards,
Daniel
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