On Fri, May 12, 2017 at 06:53:44PM +0200, Kevin Wolf wrote: > 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. Okay, I didn't know you and Rik had already discussed this in private. The QEMU change is probably not difficult. Stefan
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