Re: KVM "fake DAX" device flushing

Rik van Riel
2017-05-11T21:36:05Z

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On Thu, 2017-05-11 at 12:15 -0700, Dan Williams wrote:
> On Thu, May 11, 2017 at 11:17 AM, Stefan Hajnoczi <stefanha@redhat.co
> m> wrote:
> > On Wed, May 10, 2017 at 09:26:00PM +0530, Pankaj Gupta wrote:
> > > We are sharing initial project proposal for
> > > 'KVM "fake DAX" device flushing' project for feedback.
> > > Got the idea during discussion with 'Rik van Riel'.
> >
> > CCing NVDIMM folks.
> >
> > >
> > > Also, request answers to 'Questions' section.
> > >
> > > Abstract :
> > > ----------
> > > Project idea is to use fake persistent memory with direct
> > > access(DAX) in virtual machines. Overall goal of project
> > > is to increase the number of virtual machines that can be
> > > run on a physical machine, in order to increase the density
> > > of customer virtual machines.
> > >
> > > The idea is to avoid the guest page cache, and minimize the
> > > memory footprint of virtual machines. By presenting a disk
> > > image as a nvdimm direct access (DAX) memory region in a
> > > virtual machine, the guest OS can avoid using page cache
> > > memory for most file accesses.
>
> How is this different than the solution that Clear Containers came up
> with?
>
> https://lwn.net/Articles/644675/

Clear Containers uses MAP_PRIVATE with read-only
images.

This solution is about making read-write images
work. When a program in the guest calls fsync,
we need to ensure the data has actually hit the
disk on the host side before fsync returns.

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