Re: [Xen-users] Error: Device 769 (vbd) could not be connected...Limit of 25 domains. SOLVED.

[Thomas Karsten <thomas@xxxxxxxxxxx>]

Hallo,

thanks for your suggestion.

On Thursday 20 July 2006 18:15, you wrote:
> This is a known issue.
> It has been fixed in xen unstable.
> The problem is loop device is not leased after the guest destroyed.
> If execute "losetup -d /dev/loopX" in domain0, the problem will be solved 
> temporarily.

Since I have 255 loopback devices installed in my system, I am not running
out of loopback devices (because this limitation also occures when starting
the domUs for the first time).

However, after some testing I manage the handling of the loopback devices on
my own now and pass each loopback device (physical device) to Xen rather than
the image file (file backed vbd):

        for ((i=10; i < 130; i++)); do
            loop_device=$(losetup -f)
            losetup $loop_device /xen-images/guestdom1.img
            xm create /etc/xen/guestdoms \
                    name="guestdom$i" \
                    dev="phy:$loop_device,sda1,ro"
        done

This works well and it does not end up in the error message. Of course, I also
have to take care of detaching the image from each loopback device after the
guest shut down.

Using this method I am able to create up to 111 concurrent running domUs,
afterwards I get the error message "Error: (12, 'Cannot allocate memory')".
This is a known limitation caused by Xen's internal heap of 16 MB
(http://lists.xensource.com/archives/html/xen-devel/2006-04/msg00321.html).

I just wonder, why I get a problem using the file backed vbds, but why I
don't run into this problem using the physical device. I thought that Xen
internally also attaches the image file to a free loopback device. If this
was the case, then why can Xen create so many domUs when physical devices are
passed to it, but has a problem with using file backed vbd? Any ideas for
this?

By the way, I also found out that Xen will create a guest faster when passing
the physical device to it than just passing the image file.

Thanks,
Thomas

> >On Wednesday 19 July 2006 18:18, Thomas Karsten wrote:
> >> Hello,
> >>
> >> I am running Xen 3.0.2 in an Ubuntu 6.06 Linux system (kernel
> >> 2.6.16-xen). Xend is running and works well.
> >>
> >> I would like to use Xen to run about 80 domains (for testing purpose).
> >> All domains share the same image file (which therefor is mounted
> >> 'readonly' by Xen) and each domain is configured to use only 10 MB of
> >> RAM. I am using a script to generate name, hostname and IP address for
> >> each guest (it simply loops over i and calls:'xm create name=guestdom$i
> >> hostname=$i ip=192.168.1. $i').
> >>
> >> Everything works fine, but after 25 concurrent running domains Xen
> >> refuses to create any further domains, giving me the error:
> >>
> >> Error: Device 769 (vbd) could not be connected. Hotplug scripts not
> >> working.
> >>
> >> On this mailing list I found several threads dealing with this error
> >> message. However, I did not find that anybody is facing this kind of
> >> limitation that I have. For your information, the number of loopback
> >> devices in my system is 132. Also, I am not running out of memory, this
> >> would be stated by Xen (I have 1500 MB of RAM, where 500 MB are
> >> occupied by Xen).
> >>
> >> I can remove a guest domain and create a new one, this is no problem.
> >> However, on my system I am not able to create more than 25 domains at
> >> the same time. Can anybody help please?
> >>
> >> Thank you very much, all ideas are appreciated,
> >> Thomas
> >
> >_______________________________________________
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> >Xen-users@xxxxxxxxxxxxxxxxxxx
> >http://lists.xensource.com/xen-users
> 

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