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Re: [ovirt-users] [OT] Major and minor numbers assigned to /dev/vdx virt

From: Gianluca Cecchi
Subject: Re: [ovirt-users] [OT] Major and minor numbers assigned to /dev/vdx virtio devices
Date: Tue, 14 Jul 2020 17:03:28 +0200

On Tue, Jul 14, 2020 at 4:40 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
On Mon, Jul 13, 2020 at 09:53:31PM +0300, Nir Soffer wrote:
> On Wed, Jul 1, 2020 at 5:55 PM Gianluca Cecchi
> <gianluca.cecchi@gmail.com> wrote:
> >
> > Hello,
> > isn't there an official major/minor numbering scheme for virtio disks?
> > Sometimes I see 251 major or 252 or so... what is the udev assignment logic?
> > Reading here:
> > https://www.kernel.org/doc/Documentation/admin-guide/devices.txt
> >
> >  240-254 block LOCAL/EXPERIMENTAL USE
> > Allocated for local/experimental use.  For devices not
> > assigned official numbers, these ranges should be
> > used in order to avoid conflicting with future assignments.
> >
> > it seems they are in the range of experimental ones, while for example Xen /dev/xvdx devices have their own static assignment (202 major)

No, the Linux virtio_blk driver does not use a static device major number.

Regarding udev, on my Fedora system
/usr/lib/udev/rules.d/60-persistent-storage.rules has rules like this:

KERNEL=="vd*[!0-9]", ATTRS{serial}=="?*", ENV{ID_SERIAL}="$attr{serial}", SYMLINK+="disk/by-id/virtio-$env{ID_SERIAL}"

The rules match on the "vd*" name. If you are writing udev rules you
could use the same approach.

Is there a specific problem faced when there is no static device major


Thanks for the information.
No, it was only a curiosity: during a recovery action (actually it was a "poor man" P2V operation using dd) where I had to rebuild initrd file and to reinstall grub in a chroot environment,
I had to run mknod commands to manually create the /dev/vdax files and comparing two different existing guests I stumbled upon their /dev/vda files that had different major/minor numbers, so I was not sure what to use....


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