[libvirt] Unable to create cgroup for driver

[Daniel P. Berrange]

On Tue, Aug 11, 2009 at 06:04:29PM +0900, Jun Koi wrote:
> On Tue, Aug 11, 2009 at 6:00 PM, Daniel P. Berrange<berrange at redhat.com> wrote:
> > On Tue, Aug 11, 2009 at 10:03:32AM +0900, Jun Koi wrote:
> >> Hi,
> >>
> >> The libvirt 0.7 has a following error when i start libvirtd:
> >>
> >> warning : qemudStartup:521 : Unable to create cgroup for driver: No
> >> such device or address
> >
> > That is not an error, it is a expected warning if you don't have any
> > cgroups mount. It is designed to continue working
> >
> 
> Could you explain what is that cgroups mount, and why I need that??

Cgroups are principally a resource control mechanism for processes.
There are countless tunables, and a libvirt will use certain ones,
specifically for QEMU it currently uses the 'cpu' and 'devices'
controllers, while LXC also uses 'memory' and 'cpuacct'.

You can activate it by mounting the desired controllers before libvirtd
starts. eg

   mkdir /dev/cgroup
   mount -t cgroup none /dev/cgroup -o cpu,memory,devices

> The fact is that I never had this problem before (with older git version)

That's because this is a new feature. It is optional, hence why it is
merely a warning message, and not an error message.

> >> Then it fails to start.
> >>
> >> How can I fix this?
> >
> > You'll need to provide more information than this. What exactly fails
> > to start ? The entire daemon, just the QEMU driver, or something else ?
> > Does it exit, does it crash ? Get a debug log when it fails, etc
> 
> When I run "libvirtd -v", it simply reports error like:
> 
> "warning : qemudStartup:521 : Unable to create cgroup for driver: No
> such device or address
> Segmentation fault"

Can you capture a stack trace in GDB, and/or get the debug logs from
running with LIBVIRT_DEBUG=1. There could be a thousand & one other
things being done between the time the warning message is printed, and
it actually crashes, so we can't assume they're neccessarily related

Daniel
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