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Re: [libvirt] PATCH: Support setting a disk drive serial (number)

On Mon, Aug 17, 2009 at 02:37:48PM -0400, Dave Allan wrote:
> Daniel P. Berrange wrote:
> >All hard disks (IDE, SCSI, USB, VirtIO) have a concept of a serial number
> >which is intended to uniquely identify them. If you mean this off, QEMU
> >just makes up a serial on the fly. THis is not guarenteed to be stable
> >across guest reboots with hardware changes. It is thus desirable to be
> >able to specify this in libvirt XML. To that end, this patch allows
> >for a <serial> element inside a disk
> >
> >    <disk type='file' device='disk'>
> >      <source 
> >      file='/media/lacie-500-disk-2/virtual-machines/scratch5.img'/>
> >      <target dev='vda' bus='virtio'/>
> >      <serial>dan123virtio</serial>
> >    </disk>
> >
> >the contents is free-form. It implemenmts this for the QEMU driver for
> >any disk using -drive style args. Unfortunately this excludes USB disk,
> >even though internally QEMU can set a serial for these, the -usbdevice
> >syntax does not allow it
> >
> >Daniel
> >
> >* docs/schemas/domain.rng: Add <serial> element to disks
> >* src/domain_conf.h, src/domain_conf.c: XML parsing and
> >  formatting for disk serial numbers
> >* src/qemu_conf.c: Set serial number when launching guests
> >* tests/qemuxml2argvdata/qemuxml2argv-disk-drive-shared.args,
> >  tests/qemuxml2argvdata/qemuxml2argv-disk-drive-shared.xml: Add
> >  serial number to XML test


> This patch seems like a very good idea to me.  I'm surprised it doesn't 
> cause more grief if the serial # keeps changing.  I didn't review the 
> code too closely, but a definite ACK to the functionality, and I saw no 
> problems with the implementation in the quick look I took.

Well it causes grief for Windows guests with WHQL'd virtio drivers I
believe. We've probably just been fortunate with Linux guests that not
much functional stuff is based off the serial number. The serial number
is used for /dev/disk/by-id symlinks, but most distros use /dev/disk/by-uuid 
symlinks for mounting volume instead and that's based off a filesystem
level UUID, instead of block device level serial.
> As a possible additional feature, in the case of an entire device being 
> assigned to the guest, have you considered passing through the serial of 
> the underlying device?  The serial is available in the nodedev XML.

That's a policy decision, and so not within scope of libvirt. Of course 
the application provisioning a new guest can see the host device's 
serial, and copy that across to the guest XML it uses if desired.

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