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Date: Mon, 25 Apr 2016 11:20:50 +0200
From: Igor Mammedov <imammedo@redhat.com>
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Subject: Re: [Qemu-devel] [RFC PATCH v2 2/2] spapr: Memory hot-unplug support
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To: Bharata B Rao <bharata@linux.vnet.ibm.com>
Cc: David Gibson <david@gibson.dropbear.id.au>, thuth@redhat.com, qemu-devel@nongnu.org, mdroth@linux.vnet.ibm.com, qemu-ppc@nongnu.org, nfont@linux.vnet.ibm.com

On Wed, 16 Mar 2016 10:11:54 +0530
Bharata B Rao <bharata@linux.vnet.ibm.com> wrote:

> On Wed, Mar 16, 2016 at 12:36:05PM +1100, David Gibson wrote:
> > On Tue, Mar 15, 2016 at 10:08:56AM +0530, Bharata B Rao wrote:  
> > > Add support to hot remove pc-dimm memory devices.
> > > 
> > > Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>  
> > 
> > Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> > 
> > Looks correct, but again, needs to wait on the PAPR change.
[...]
> 
> While we are here, I would also like to get some opinion on the real
> need for memory unplug. Is there anything that memory unplug gives us
> which memory ballooning (shrinking mem via ballooning) can't give ?
Sure ballooning can complement memory hotplug but turning it on would
effectively reduce hotplug to balloning as it would enable overcommit
capability instead of hard partitioning pc-dimms provides. So one
could just use ballooning only and not bother with hotplug at all.

On the other hand memory hotplug/unplug (at least on x86) tries
to model real hardware, thus removing need in paravirt ballooning
solution in favor of native guest support.

PS:
Guest wise, currently hot-unplug is not well supported in linux,
i.e. it's not guarantied that guest will honor unplug request
as it may pin dimm by using it as a non migratable memory. So
there is something to work on guest side to make unplug more
reliable/guarantied.

> 
> Regards,
> Bharata.
> 
>