Xen users - Nov 2011 - newbie question re block-attach on ubuntu 11.10

I''m a complete newbie at Xen, just getting started.

I was happy to see that Ubuntu 11.10 includes Xen again,
http://zulcss.wordpress.com/2011/09/04/xen-4-1-1-on-ubuntu/
So I installed ubuntu 11.10''s precompiled Xen,
and started playing around with it.
I''m not even trying to start another VM at the moment,
just playing around with mounting block devices,
and had trouble with block-attach:

$ dd if=/dev/zero of=foo.img bs=1024k seek=64 count=0
0+0 records in
0+0 records out
0 bytes (0 B) copied, 1.6493e-05 s, 0.0 kB/s
$ sudo xm block-attach 0 file://home/dank/lvmstuff/foo.img /dev/xvda1 w 0
$ sudo xm block-list 0
Vdev BE handle state evt-ch ring-ref BE-path
51713 0 0 1 -1 -1 /local/domain/0/backend/vbd/0/51713
$ sudo mkfs.ntfs /dev/xvda1
Failed to access ''/dev/xvda1'': No such file or directory
The device doesn''t exist; did you specify it correctly?

Was I supposed to create /dev/xvda1 myself? I thought Xen
did that.

Thanks,
Dan

p.s.
The little example script I''m trying to get working is
http://kegel.com/linux/xen/demo-xen.sh

On Thu, Nov 17, 2011 at 10:24 PM, Dan Kegel <dank@kegel.com>
wrote:
> $ sudo xm block-attach 0 file://home/dank/lvmstuff/foo.img /dev/xvda1 w 0
> $ sudo xm block-list 0
> Vdev BE handle state evt-ch ring-ref BE-path
> 51713 0 0 1 -1 -1 /local/domain/0/backend/vbd/0/51713
> $ sudo mkfs.ntfs /dev/xvda1
> Failed to access ''/dev/xvda1'': No such file or directory
> The device doesn''t exist; did you specify it correctly?
>
> Was I supposed to create /dev/xvda1 myself? I thought Xen
> did that.
The workaround/fix is
sudo modprobe xen-blkfront

Is that traditionally not loaded on dom0?
- Dan

For completeness, here''s a script that shows the smallest possible
example
of using xen and lvm together to create and format a blank virtual disk,
and then to tear it all down.   It works with ubuntu-11.10.
There''s nothing at all special about it, it''s just an first
example for newbies
trying to work out how volume management works,
expressed purely as a shell script that uses xm in dom0.
It assumes you''re using a 32 bit kernel.
(This is http://kegel.com/linux/xen/demo-lvm-xen.sh.txt )

#!/bin/sh
# Script to demonstrate creating and mounting a disk image using LVM and Xen
set -x
set -e

lvm version || sudo apt-get install lvm2 dmsetup mdadm

# After installing Xen, reboot and choose the Xen kernel.
# Note that you won''t be able to use nvidia graphics cards under Xen,
# so you''ll probably need to ssh into the box after booting linux in
xen.
xm help > /dev/null || sudo apt-get install xen-hypervisor-4.1-i386
xenstore-utils xen-tools xen-utils-4.1 xen-utils-common xenwatch

test -b /dev/xvda1 || sudo modprobe xen-blkfront

# Create a demo physical volume
# Make a sparse image file of size 64MB
dd if=/dev/zero of=foo.img bs=1024k seek=64 count=0
# Make a loopback device for it
sudo losetup /dev/loop0 foo.img
# Register it as a physical volume
sudo pvcreate -f -M2 /dev/loop0
# List the known physical volumes
sudo pvdisplay

# Create a demo volume group and put that physical volume in it
sudo vgcreate demo_vg /dev/loop0
# List the known volume groups
sudo vgdisplay

# Create a logical volume in that volume group
sudo lvcreate --name foo_lv --size 50MB demo_vg

# List the known logical volumes
sudo lvdisplay

# Attach it to domain 0
sudo xm block-attach 0 phy:/dev/demo_vg/foo_lv /dev/xvda1 w 0

# Show the newly attached block device
sudo xm block-list 0

# Create a filesytem on it
sudo mkfs.ntfs /dev/xvda1
# Mount it somewhere
sudo mkdir /media/foo
sudo mount /dev/xvda1 /media/foo

# Show that it''s real
df /media/foo

# Unmount it
sudo umount /media/foo
sudo rmdir /media/foo

# Detach it
sudo xm block-detach 0 /dev/xvda1 -f

# Remove the logical volume
# BUG?: in Ubuntu 11.10, lvremove complains "semop failed"
# See https://bugs.launchpad.net/ubuntu/+source/lvm2/+bug/846061
sudo lvremove -f /dev/demo_vg/foo_lv

# Remove the volume group
sudo vgremove demo_vg

# Remove the physical volume
sudo pvremove /dev/loop0

# BUG?: have to sleep here, else losetup -d fails sometimes?
sleep 1

# Remove the loopback device
sudo losetup -d /dev/loop0
# Remove the image
rm foo.img