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

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

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.
>      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.

Dan.
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