Re: [Xen-devel] [RFC PATCH] mm, hotplug: get rid of auto_online_blocks

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

Heiko Carstens <heiko.carstens@xxxxxxxxxx> writes:

> On Mon, Feb 27, 2017 at 11:02:09AM +0100, Vitaly Kuznetsov wrote:
>> A couple of other thoughts:
>> 1) Having all newly added memory online ASAP is probably what people
>> want for all virtual machines.

Sorry, obviously missed 'x86' in the above statement.

>
> This is not true for s390. On s390 we have "standby" memory that a guest
> sees and potentially may use if it sets it online. Every guest that sets
> memory offline contributes to the hypervisor's standby memory pool, while
> onlining standby memory takes memory away from the standby pool.
>
> The use-case is that a system administrator in advance knows the maximum
> size a guest will ever have and also defines how much memory should be used
> at boot time. The difference is standby memory.
>
> Auto-onlining of standby memory is the last thing we want.
>

This is actually a very good example of why we need the config
option. s390 kernels will have it disabled.

-- 
  Vitaly

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