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Re: [Xen-users] adding fourth device to pvhvm config causes DomU launch fail @ "device_disk_add: device already exists in xenstore" ?

On Mon, Feb 22, 2016, at 07:57 AM, Roger Pau Monné wrote:
> Right, using the hd* syntax you can only address four drives (this is
> the limitation in IDE IIRC). So starting from hdc, any other disk that
> you want to attach must use the xvd* syntax, and it is recommended to
> start at xvde. There's a document in the Xen tree that contains a more
> detailed explanation of all this:
> http://xenbits.xen.org/docs/unstable/misc/vbd-interface.txt

Ok, that explains it.  That one I had NOT seen.  

Reading there

"For Linux HVM guests using PV-on-HVM drivers, users are recommended to
supply as few hd* devices as possible, and for the rest of the disks,
to use pure xvd* devices starting at xvde.  Modern PV-on-HVM drivers
will map provided hd* devices to the corresponding /dev/xvd* (for
example, hda is presented also as /dev/xvda).

Some Linux HVM guests with broken PV-on-HVM drivers do not cope
properly if both hda and hdc are supplied, nor with both hda and xvda,
because they directly map the bottom 8 bits of the xenstore integer
directly to the Linux guest's device number and throw away the rest;
they can crash due to minor number clashes.  With these guests, the
workaround is not to supply problematic combinations of devices."

Is there any reason I wouldn't just set ALL of the drives to xvd*?  I.e., do I 
*need* any hd*?

I.e., what are "as possible" and "problematic combinations" actually defined as?

> Also, it seems you are passing a bunch of lvm volumes as different
> disks, do you know you can for example use hda1, hda2... and so on in
> order to setup different partitions that are presented to the guest as a
> single disk?
> http://xenbits.xen.org/docs/unstable/misc/xl-disk-configuration.txt

Yeah, aware of that.  And often do that.  Here, though, I'm 
separating/isolating for use in a (in-progress ...) backup/restore strategy.

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