Re: [Qemu-devel] KVM "fake DAX" device flushing

[Kevin Wolf]

Am 12.05.2017 um 15:42 hat Stefan Hajnoczi geschrieben:
> 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
> :).

There are other reasons why we recommend against using non-O_DIRECT
modes in production (including the error handling), but with respect to
live migration, this is the only one I'm aware of.

As I already said in the private email thread, an FADV_INVALIDATE_CACHE
should do the trick and I'd be happy to work with you guys on that.

Kevin

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