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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Tue, Apr 14, 2015 at 05:40:24PM +0800, Hongyang Yang wrote:
> 
> 
> 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.
> 

Because now the correct value is recorded during domain creation, it will
always be sent to the other end, so there is no need for this kind of
hack.

Wei.

> >
> >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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