Re: [Xen-devel] [PATCH 1/1] xc_domain_restore: Allow QEMU to increase memory

[Hongyang Yang <yanghy@xxxxxxxxxxxxxx>]

On 04/14/2015 05:29 PM, Wei Liu wrote:
> On Tue, Apr 14, 2015 at 05:22:31PM +0800, Hongyang Yang wrote:
> [...]
> > > If I understand correctly, the steps are this:
> > >
> > > * 'xl create' makes a VM of size $FOO
> > > * qemu bumps the size to $FOO+$N
> > > * 'xl save' writes $FOO+$N of page data, but the xl config file at the
> > > start of the image still says $FOO
> > > * 'xl restore' creates a vm of size $FOO, then instructs
> > > xc_domain_restore() to put $FOO+$N pages into it.
> > >
> > > I would argue first, that qemu should not play in this area to start with.
> > >
> > > However, the real bug here is that the domain configuration written by
> > > xl save is inaccurate.
> > There's a case like COLO:
> > 1. Both Primary/Secondary VM are created first with the same config file
> >     which makes a VM of size $FOO
> > 2. qemu bumps the size to $FOO+$N
> > 3. 'save' writes $FOO+$N of page data
> > 4. 'restore' put $FOO+$N pages into $FOO pages which will cause error
> >
> > Even if you fix the configuration, the bug still exists because the config
> > only been transferred from Primary to Secondary once at the very beginning
> > when you execute 'xl remus' command. The problem is how to let the secondary
> > (restore) side knows the size change? Through a migration command(which is
> > easier in v2 migration) or some other solution?
> As I said in my reply to Don, the extra memory can be saved during
> domain creation. That would solve this problem.
After migration we'll enter COLO mode, which will continously send migration
stream to Secondary. Domain creation only happens before doing live migration.

> Wei.
>
> > Form this point of view, I think Don's solution is one way to solve the
> > problem.
> >
> > > ~Andrew
> > > .
> > --
> > Thanks,
> > Yang.
> .
--
Thanks,
Yang.

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