RE: [Xen-users] "xm save" hanging when saving domain in "pause" state

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

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From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Claris Castillo
Sent: 15 July 2006 00:12
To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] "xm save" hanging when saving domain in "pause" state

Thanks for your reply Michael. These are bad news... I am actually looking at options to do offline migration, that is, take snapshots of the VMs state and their corresponding filesystems in order to be able to fire them up in a different host if for some reason the original host they are assigned to crashes at some point. I have found some threads with *very* similar questions in some mailing lists, unfortunately they don't extend to more than two entries and have more questions than answers.

 

 

Ok. Parameter of design? As far as I understand by "pausing" a domain, XEN is basically indicating the scheduler not to give any other slice to that particular VM from that point in time (until the VM is unpaused)  Why can't XEN0 just surpass the scheduler, indicate to the VM that it must disconnect its devices (basically doing an enhanced version of "xm unpause") and put itself in quiescent state, and wait to be notified by the VM (step 2 in your email)?. What is wrong with such approach? Am I missing something? Would not this enable a clean checkpointing procedure?

Thanks

cc 

 

cc, I think you could do exactly what you want with JUST xm save, it automatically "pauses" the domain. Is there any particular reason you CAN'T do that? Why do you need to do xm pause before xm save?

 

--

Mats

 

On 7/14/06, Michael Vrable <mvrable@xxxxxxxxxxx> wrote:

On Thu, Jul 13, 2006 at 10:32:47PM -0700, Claris Castillo wrote:
> Is there any known issue on saving a domain which is in pause state?
> Xen hangs whenever I try to save (by means of xm save command) the state of
> a machine which has been paused (by means of xm pause command). I have been
> looking at the log files etc but I am not able to spot the problem. BTW, xm
> save works prefectly fine if the domain is running or block. The problem is
> when the domain is in pause.

Yes, this is by design.  Save/restore are a cooperative process:
1. Xend notifies the domain that it will be saved
2. The domain disconnects from devices, places itself into a quiescent
   state, and notifies Xen
3. After receiving this notification, xend saves the domain's CPU state
   and memory to a file
Pausing a domain will prevent any progress from being made on step 2
(for the obvious reason--the domain can't run), so saving will hang.

--Michael Vrable

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--
Claris Castillo
http://www4.ncsu.edu/~ccastil
PhD. Candidate
Computer Science
North Carolina State University
Raleigh, NC

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