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

To: "Paolo Supino" <paolo@xxxxxxxxxxxxx>
Subject: Re: [Xen-users] Overcoming loopback device limitation - revisited
From: "john maclean" <jayeola@xxxxxxxxx>
Date: Mon, 31 Jul 2006 10:31:34 +0100
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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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