Re: [Xen-users] Zombie VMs cannot be destroyed

[Tim Post <tim.post@xxxxxxxxxxxxxxx>]

On Fri, 2006-12-01 at 14:44 -0500, Michael Froh wrote:
> On 1-Dec-06, at 1:25 PM, Tim Post wrote:
> 

> 
> Understood.  Right now I'm just playing with Xen so there is no data  
> to be lost.
> 

<phew>

> The domains which were correctly destroyed were centos domainU with
> ext3 fs mounted.  These are snapshots of a pristine ext3 fs so will just
> recreate the snapshots in my xen sandbox.
> 
> As noted in the list below, the remaining were dsl & knoppix domainU  
> which
> only had their respective .iso images mounted ro so inode flushing  
> should
> not be a problem here.
> >

Its hard to tell what was causing the to zombie.. if its just the HVM
guests that were hanging it tells me their BIOS had some unfinished
business.

XM destroy does just what it says, destroys a guest in memory and (tries
to) instantly free any associated bios block devices and memory. If you
do a xm shutdown [domname] and keep typing xm list while waiting for it
to happen, you'll notice most full or paravirt guests zombie for just a
second.

> >> This destroyed all of the para-virtualized domains running (4 of
> >> them) but turned all the HVM VMs into Zombies as shown here:
> >>
> >

Most likely the virtuailzed BIOS, but no idea why exactly.


> Thanks for the draft script.  I haven't gotten around to playing with  
> xendomains
> yet, but will.
> 
> I have since tried to recreate the problem but have been unable to  
> after a
> system reboot.  I have tried 24 running domains and all "destroy"  
> properly,
> so it seems it wasn't a timing issue.

You may want to share this with xen-devel. While you are doing the
opposite of whats appropriate, they may be interested to see your
experience.

I'm trying to think of everything in a guest that could have '
unfinished business ' that looped forever and the only thing coming to
mind is the BIOS.

Does this happen with just one or two hvm guests as opposed to 10+? ..
dom-0 expects a certain amount to return to heap. I'm not sure if shadow
paging specified for a fully virt VM's BIOS is treated any differently
than conventional paging for a guest when it comes to xm destroy either,
a better question for xen-devel

What I do know is dom-0 did not get back what it gave the guest prior to
destroying it. Were you using rocks or some other kind of socket helper?
How many nics?

I'm trying to duplicate what you did without much luck, but I don't have
any Intel platform to do it with. Its not duplicating on my X2.

> Tim, when you do get zombie domains, how do you eventually purge them
> since they seem to be using memory but no CPU.
> >

Typically they vanish within a second or two. Again, "destroy" is not at
all the best way to stop a guest, usually its used only if a guest
doesn't respond to an orderly shutdown. 

xm shutdown = shutdown -f now

xm destroy  = yank out the power cord... actually its more like open the
case, yank out the HD, then the ram, then the BIOS (if hvm) then the PSU
(and by extension the power cord) while still running. Not sure if I got
them in the right order but I think you get the gist :) Not what you'd
want to do in production at all, especially with no substantial pause in
between.

> >> Mike.

Best,
-Tim



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