Re: [Xen-devel] [RFC Design Doc] Add vNVDIMM support for Xen

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> Sent: Tuesday, February 02, 2016 2:25 AM
> 
> On 01/02/16 05:44, Haozhong Zhang wrote:
> > Hi,
> >
> > The following document describes the design of adding vNVDIMM support
> > for Xen. Any comments are welcome.
> >
> > Thanks,
> > Haozhong
> 
> Thankyou for doing this.  It is a very comprehensive document, and a
> fantastic example for future similar situations.

Agree. It's a very good doc to help following discussions.

> 
> 
> To start with however, I would like to clear up my confusion over the
> the usecases of pmem vs pblk.
> 
> pblk, using indirect access, is less efficient than pmem.  NVDIMMs
> themselves are slower (and presumably more expensive) than equivalent
> RAM, and presumably still has a finite number of write cycles,  so I
> don't buy an argument suggesting that they are a plausible replacement
> for real RAM.

for pblk, I think it's more meaningful to compare with today's SSD.

> 
> I presume therefore that a system would only choose to use pblk mode in
> situations where the host physical address space is a limiting factor.
> Are there other situations which I have overlooked?

I think you more mean RAM size limitation here. Today 48bit physical
address space is not limiting yet. :-)

> 
> Secondly, I presume that pmem vs pblk will be a firmware decision and
> fixed from the point of view of the Operating System?
> 

Yes from the description of ACPI manner. 

Thanks
Kevin

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