Re: [Xen-users] Overcoming loopback device limitation - revisited

["john maclean" <jayeola@xxxxxxxxx>]

Thanks Paolo,

You are correct with the info for creating loopback devices. I made a
few ealier with the following:-

mkdir -p /xen/images
for X in $(seq 0 9) do;
   dd if=/zev/zero of=/xen/images/vm0$.img bs=1024k count=1024
   dd if=/zev/zero of=/xen/images/vm0$-swap.img bs=1024k count=128
   mkfs.ext3 /xen/images/vm0$.img
   mkswap /xen/images/vm0$-swap.img
done

I then mount -o loop /xen/images/vm-foo.img /mnt/blah
Populate it with an OS (e.g. Debian, Gentoo....)
Configure domU, umount and fire up. I'll destroy the *one* domU that's
running, create spme more loopback devices and see what happens.

I'll not reboot, restart or reinstall anything! I actually want to
learn how this works!



On 31/07/06, Paolo Supino <paolo@xxxxxxxxxxxxx> wrote:
> Hi John
>
>  I got stuck with a similar problem in the past :-( Before I reply to
> your question here is something  learned about loop devices and Xen:
> Have the kernel be able to manage the same number of loop devices you
> have /dev directory. No less, no more (otherwise some unexpected things
> happen). Now to your questions:
> 1. A loop filesystem is nothing special. From the point of view of Dom0
> it is nothing more than a file and since it nothing more than a file you
> create it using the command: `dd if=/dev/zero of=[filename] bs=Yk
> count=X (Y is a number and k stands for Kilobytes, X is also a number).
> Do note that block size is for DD only and has no implication for the
> block size used inside by the filesystem. After the file is created you
> simply run `mkfs -t [filesystem type] [filename]` to create the
> filesystem inside(mkfs will nag you about the file not being a block
> special device, answer yes to the proceed question). Last step is to
> mount the newly created filesystem somewhere in Dom0 with the command
> `mount -o loop [filename] [mount point] and populate it.
> 2. The commands `dd`, mkfs and mount are always installed in Linux you
> should be OK. If you don't have one of them something is very wrong with
> your installation. One important thing to note: `mkfs` is only a wrapper
> to the filesystem specific `mkfs`. Make sure that you have the binaries
> for the chosen filesystem.
> 3. Your loop devices and permissions seem all in order (I have the same
> exact configuration and it works).
> 4. About udev mojo: I have no clue, I don't use udev (I hate it and
> refuse to use it ;-))
>
>
>
>
>
> Paolo
>
>
> john maclean wrote:
>
> > Hi there chaps,
> >
> > Just to get me up and running I ran `make world && make install` from
> > source. All went well until I started running out of loopback devices
> > to create guests. I know of the 8 devices limitation and I'm sure that
> > the loop "module" is built into the kernel;
> >
> > modinfo loop
> > filename:       /lib/modules/2.6.16-xen/kernel/drivers/block/loop.ko
> > license:        GPL
> > alias:          block-major-7-*
> > vermagic:       2.6.16-xen SMP 686 gcc-3.3
> > depends:
> > parm:           max_loop:Maximum number of loop devices (1-256) (int)
> >
> > OK, that number is more than enough for me. This is a laptop and I'd
> > want to manage about 12. So I use this boot time parameter even though
> > I know/think that it's a built in:-
> >
> > cat /proc/cmdline
> > root=/dev/hda3 ro console=tty0 max_loop=32
> >
> > To create some more loop back devices I ran the following:-
> >
> > for X in $(seq 8 31); do mknod ;  MAKEDEV /dev/loop$X  b 7 $i ; chmod
> > 660 /dev/loop$X; chown 0.disk /dev/loop$X;  done
> >
> > Now I can only seem to have just one domU running!
> >
> > #acid~#  xm list
> > Name                              ID Mem(MiB) VCPUs State  Time(s)
> > Domain-0                           0      512     1 r-----   108.4
> > vm01                               1       64     1 ------     0.3
> >
> > acid:~# xm create vm00 -c /etc/xen/vm00
> > Using config file "/etc/xen/vm00".
> > Error: Device 769 (vbd) could not be connected. Backend device not found.
> >
> > I have a few simple-ish questions that I'd a appreciate some answers for;
> > - What is the file system that creates loopback devices a required,
> > on the fly?
> > - How do I know if I have this?
> > - Did I give the newly created devices the wrong permissons?
> > - Why does it appear that all of the /dev/loop devices bar one are
> > "blocked"?
> > - Do I have to do some /etc/udev mojo?
> >
> > version:- xen-3.0.2-2
> > arch:- debian sarge
> > me:- very stuck ;)
--
John Maclean  - 07739 171 531
MSc (DIC)

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