Re: [Xen-devel] xen-vbd interface (segment size expansion) - FreeBSD host have it.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Thu, Oct 18, 2012 at 01:59:36AM +0000, Duan, Ronghui wrote:
> Hi, I am back from a long holiday. Sorry for the delay replay.
> > I was wondering how the protocol you developed works when it comes to
> > migration to a host that does not support the new features?
> >
> For my part, when VM migrate to a host which do not support larger segments, 
> frontend will fall back to use original protocol with old-segments 11.
> 
> > Specifically how do deal with a guest which tries to replay in progress I/Os
> > that do not fit within the old-segment size (11)?
> The in-progress bio which have larger segments > 11 will receive an io error. 
> I do not find a better way to handle it yet.

Justin, how did you guys handle it in FreeBSD? Or is it dependent on
the backends always supporting these large segments?

thanks!
> 
> > -----Original Message-----
> > From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx]
> > Sent: Tuesday, September 25, 2012 11:26 PM
> > To: Justin Gibbs; xen-devel@xxxxxxxxxxxxxxxxxxx
> > Cc: Duan, Ronghui
> > Subject: Re: xen-vbd interface (segment size expansion) - FreeBSD host have
> > it.
> > 
> > On Mon, Aug 20, 2012 at 06:56:02PM +0000, Justin Gibbs wrote:
> > > Ronghui,
> > >
> > > It has been a while since I've actively worked on the blkif stuff.  ...
> > >
> > > That said, I'm happy to help in whatever ways I can to help get the blkif
> > interface sorted out.  I see several steps that should be taken:
> > >
> > > 1) Fix the Xen and QEMU builds so that QEMU keeps its own copy of the
> > Xen interface version.  This will allow interfaces to rev safely and in a
> > coordinated fashion (i.e. update interface in Xen, then add support for the
> > new interface in QEMU upstream).
> > >
> > > 2) Complete support in drivers for the existing blkif interface so that 
> > > you
> > get maximum performance against systems using the existing multi-page
> > extensions.
> > >
> > > 3) Do something to allow for larger and more numerous requests.
> > >
> > > On point 3, my approach was to try to perturb the existing protocol as 
> > > little
> > as possible in the hopes that other implementations could quickly be
> > enhanced to support the feature.  However, there is a lot of ugliness in the
> > existing blkif interface.  I can certainly understand the desires of some 
> > to just
> > replace blkif with a blkif2.
> > >
> > > What are your current plans in this area?  How can I be of assistance?
> > 
> > Hey Justin and Ronghui,
> > 
> > Note: I've expanded the email thread to include xen-devel.
> > 
> > I was wondering how the protocol you developed works when it comes to
> > migration to a host that does not support the new features?
> > 
> > Specifically how do deal with a guest which tries to replay in progress I/Os
> > that do not fit within the old-segment size (11)?

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