Re: [Qemu-devel] Qemu savevm and CPU soft lockup

[Nathan Baum]

On Wed, 2009-09-23 at 20:52 +0200, Ben Accelance wrote:
> 
> 
> Le 23 sept. 2009 à 20:19, Jamie Lokier <address@hidden> a écrit :
> 
> > Benjamin Cleyet-Marrel wrote:
> >>
> >>   Hi,
> >>   After further investigation, I figured out that when issuing a  
> >> savevm
> >>   command
> >>   the entire qemu process gets stuck on IO wait.
> >>   I can't issue any other commands on the monitor the process is  
> >> shown
> >>   as D and the guest is in softlockup state.
> >>   Looking at the way migrate (and the -d for detach I presume) is
> >>   working I suppose the same behaviour would be expected from the  
> >> savevm
> >>   function.
> >>   A savevm -d so that the qemu process would not be freezed while  
> >> saving
> >>   the data.
> >>   Sorry if I am just talking non sense but my snapshot on iscsi  
> >> storage
> >>   takes about 1 minutes.
> >>   which means that my guest are down for a minute or so which is not
> >>   ideal.
> >
> > This is normal savevm behaviour, and it is exactly the reason why
> > migrate-to-file is useful.  I would not be surprised if savevm is
> > changed to use migrate-to-file internally at some point, but it does
> > not look like happening soon.
> >
> > You might avoid the guest softlockup state by stopping the guest
> > ("stop" command) before savevm, and "cont" afterwards?
> >
> > Or the guest might get just as confused, as the clock still advances.
> >
> > -- Jamie
> >
> >
> Thanks for this, at least it clarifies things. Now sorry for the  
> pain , but how do you migrate to a file and then how do you restore  
> from the file ?
> Thanks
> Cheers

The way I migrate-to-file is with

  (qemu) migrate -d "exec:cat>file".

and then restore with

  $ qemu ... -incoming "exec:cat file"