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

["Duan, Ronghui" <ronghui.duan@xxxxxxxxx>]

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.

> -----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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