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

["john maclean" <jayeola@xxxxxxxxx>]

still no joy. Managed to bring some domUs back to life though. Only
had one, now I got four.

acid:~# losetup -f
losetup: could not find any free loop device
acid:~# xm list
Name                              ID Mem(MiB) VCPUs State  Time(s)
Domain-0                           0      512     1 r-----    64.6
centos0                            1       64     1 -b----     0.1
centos1                            2       64     1 -b----     0.1
centos2                            3       64     1 -b----     0.1
centos3                            4       64     1 -b----     0.1
acid:~# cat /proc/cmdline
root=/dev/hda3 ro console=tty0 max_loop=64

---------- Forwarded message ----------
From: Paolo Supino <paolo@xxxxxxxxxxxxx>
Date: 31-Jul-2006 16:27
Subject: Re: [Xen-users] Overcoming loopback device limitation - revisited
To: john maclean <jayeola@xxxxxxxxx>


Hi John

 After 12 years of using Linux (of which 10 administrating) I learned
that there is no reason to reboot a system. And people suggesting a
reboot simply don't know what the problem is or how to solve it.


TIA
Paolo

PS - Actually there is one more reason for rebooting: Kernel upgrades
... ;-)





john maclean wrote:

> 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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