Re: [Xen-users] Xen live migration dom0 memory

[Epiontis IT <it_mailinglists@xxxxxxxxxxxx>]

On 07/01/2015 04:30 PM, Ian Campbell wrote:
> On Wed, 2015-07-01 at 16:20 +0200, Epiontis IT wrote:
> > Hello,
> >
> > I'm new to XEN live migration. I have a question regarding XEN memory
> > management while doing a vm live migration. I observed that migrating a
> > vm on XEN 4.5.1 and Linux Kernel 4.1.0 requires the dom0 on the target
> > machine to have memory available that is the size of the memory
> > allocated to the vm (2G). Since I follow "XEN best practices" and set
> > dom0_mem to 512M the target machine begins swapping out 2G to disk which
> > makes everything nearly unresponsive. Is this requirement for the dom0
> > memory size intended or might there be an issue with my setup?
> I don't think you should need 2G dom0 memory, you should need 2G of free
> host memory.
>
> Perhaps something on your destination host is thinking it wants to
> balloon dom0 down in order to make that free host memory available?
>
> Ian.
I looked further into the issue. When I start a migration with "xl 
migrate <vm> <destination>" the destination machine sets up a vm 
"<vm>--incoming" and seems to sync memory because "xl list" shows "Mem" 
for the incoming vm going up stepwise until 2048. After that though the 
vm doesn't get launched. The vm is frozen for about a minute, the dom0 
begins swapping out the 2GB to disk (because it only has 512M available 
for itself) and a process
/usr/lib/xen-4.5/bin/libxl-save-helper --restore-domain

takes about 90% of the dom0 memory. After that the vm ist launched, 
memory consumption goes back to normal, swap space is freed.
Is this some kind of cold migration process? Is there some option I'm 
missing that does real live migration?
Alex

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