[PATCH 1/1] formatdomain.html.in: mention pSeries NVDIMM 'align down' mechanic

[Daniel Henrique Barboza]

Hi,

On 7/22/20 5:05 AM, Andrea Bolognani wrote:
> On Mon, 2020-07-20 at 13:51 -0300, Daniel Henrique Barboza wrote:
>> +              <li>the remaining size (total-size - label-size), also called guest
>> +                area, will be aligned to 4KiB as default. For pSeries guests, the
>> +                guest area will be aligned down to 256MiB, and the minimum size
>> +                of the guest area must be at least 256MiB plus the label-size.</li>
> 
> The last part of the sentence seems contradictory: earlier you define
> the guest area to be the total size minus the label size, but then
> you say that the guest area must be at least 256MiB plus the label
> size.
> 
> I suggest remove the trailing "plus the label-size". Are you okay
> with me doing that before pushing? If so, then
> 
>    Reviewed-by: Andrea Bolognani <abologna at redhat.com>


I'm ok with it. Thanks for amending it!

> 
> Anyway, while it's good to have these requirements documented, I
> believe we need to rethink our approach to NVDIMMs a bit, as the
> current behavior is confusing and full of potential pitfalls.
> 
> Ideally, we'd drop the automatic alignment step which happens at
> guest startup time and replace it with an upfront check on the
> constraints: if the user tries to use a NVDIMM module that's not
> sized or aligned properly, they'd get an error telling them that the
> value they're trying to use is not good and suggesting a rounded up
> alternative.

This is similar with the approach I was taking in earlier versions of the
feature. The thing that I wasn't doing was suggesting an aligned
alternative, which makes all the difference when you think of it.


> 
> This way the size seen in the guest XML will reflect the one used at
> runtime, which is much less confusing than the existin behavior, and
> also the user will be able to create the backing file for the NVDIMM
> to be the exact size it needs to be instead of oversizing it.
> 
> If we can't get this to work without impacting existing guests, then
> at the very least we should try to reflect the rounded down size in
> the inactive guest XML instead of waiting for guest startup. This is
> not as nice as the alternative, because it requires the user to
> actively seek the adjuste value, but still better than what we have
> right now.
> 


This looks good to me. I also suggest that we take this approach solely
on pSeries guests. x86 has a way shorter alignment (2MiB vs 256MiB) in
a sense that the user isn't even aware of the automatic rounding down.


Thanks,


DHB