Re: [Xen-devel] Greater than 16 xvd devices for blkfront

[Chris Wright <chrisw@xxxxxxxxxxxx>]

* Daniel P. Berrange (berrange@xxxxxxxxxx) wrote:
> On Tue, May 06, 2008 at 01:36:05PM -0400, Chris Lalancette wrote:
> > All,
> >      We've had a number of requests to increase the number of xvd devices 
> > that a
> > PV guest can have.  Currently, if you try to connect > 16 disks, you get an
> > error from xend.  The problem ends up being that both xend and blkfront 
> > assume
> > that for dev_t, major/minor is 8 bits each, where in fact there are 
> > actually 10
> > bits for major and 22 bits for minor.

Just a nit, it's actually 12:20.

> >      Therefore, it shouldn't really be a problem giving lots of disks to 
> > guests.
> >  The problem is in backwards compatibility, and the details.  What I am
> > initially proposing to do is to leave things where they are for 
> > /dev/xvd[a-p];
> > that is, still put the xenstore entries in the same place, and use 8 bits 
> > for
> > the major and 8 bits for the minor.  For anything above that, we would end 
> > up
> > putting the xenstore entry in a different place, and pushing the major into 
> > the
> > top 10 bits (leaving the bottom 22 bits for the minor); that way old guests
> > won't fire when the entry is added, and we will add code to newer guests
> > blkfront so that they will fire when they see that entry.  Does anyone see 
> > any
> > problems with this setup, or have any ideas how to do it better?
> 
> Putting the xenstore entries in a different place is a non-starter. Too
> many things look at that location already. When blktap was added and it
> put xenstore entries in a different place it took months to track down 
> all the bugs this caused.

I'm not sure what you mean?  Since this is blkfront it'd be more like
adding a virtual-device2 to extend the protocol.

/* FIXME: Use dynamic device id if this is not set. */
err = xenbus_scanf(XBT_NIL, dev->nodename, "virtual-device", "%i", &vdevice);
if (err != 1) {
        xenbus_dev_fatal(dev, err, "reading virtual-device");
        return err;
}


IOW smth simple like:

err = xenbus_scanf(XBT_NIL, dev->nodename, "virtual-device", "%i", &vdevice);
if (err == -ENOENT)
        err = xenbus_scanf(XBT_NIL, dev->nodename, "virtual-device2", "%i", 
&vdevice);

Then we can stop propagating the myth that dev_t is 8:8.

thanks,
-chris

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