Re: KVM "fake DAX" device flushing

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, May 11, 2017 at 05:38:40PM -0400, Rik van Riel wrote:
> On Thu, 2017-05-11 at 14:17 -0400, Stefan Hajnoczi wrote:
> > On Wed, May 10, 2017 at 09:26:00PM +0530, Pankaj Gupta wrote:
> > > * For live migration use case, if host side backing file is 
> > >   shared storage, we need to flush the page cache for the disk 
> > >   image at the destination (new fadvise interface,
> > > FADV_INVALIDATE_CACHE?) 
> > >   before starting execution of the guest on the destination host.
> > 
> > Good point.  QEMU currently only supports live migration with
> > O_DIRECT.
> > I think the problem was that userspace cannot guarantee consistency
> > in
> > the general case.  If you find a solution to this problem for fake
> > NVDIMM then maybe the QEMU block layer can also begin supporting live
> > migration with buffered I/O.
> 
> I'll be happy to work with you on that, independently
> of Pankaj's project.
> 
> It looks like the fadvise system call could be extended
> pretty easily with an FADV_INVALIDATE_CACHE command, the
> other side of which can simply hook into the existing
> page cache invalidation code in the kernel.
> 
> Qemu will need to know whether the invalidation succeeded,
> but that is something we can test for pretty easily before
> returning to userspace.

Sounds great.  I will review the long discussions that took place on
qemu-devel about cache invalidation for live migration - just want to
make sure there were no other reasons why only O_DIRECT is supported :).

Stefan

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