Re: [Qemu-devel] [PATCH v2] virtio-blk: add default serial id

[Stefan Hajnoczi]

On Wed, Oct 10, 2012 at 10:07:01AM +0800, Dave Young wrote:
> On 10/09/2012 04:31 PM, Stefan Hajnoczi wrote:
> 
> > On Tue, Oct 09, 2012 at 10:27:26AM +0800, Dave Young wrote:
> >> On 10/05/2012 04:14 PM, Stefan Hajnoczi wrote:
> >>
> >>> On Sun, Sep 23, 2012 at 10:37:09AM +0800, Dave Young wrote:
> >>>> For the serial number decreasing issue, I think there's only these two 
> >>>> ways to
> >>>> select, there's no ideal way to resolve this issue.
> >>>> My use case for this is for the kdump kernel to find proper disks,
> >>>> after 1st kernel crashing 2nd kernel need find right disk to dump vmcore.
> >>>> In this case v1 and v2 aproaches are both find to me.
> >>>>
> >>>> From my point of view, patch v1 is better though, I think unpluging 
> >>>> 100000 is
> >>>> not a sane use case. It's not likely to happen.
> >>>
> >>> I'm not sure auto-assigning serial numbers is a good idea.  The guest can 
> >>> use
> >>> the serial number in /etc/fstab or other places where it expects the 
> >>> serial
> >>> number to be persistent.
> >>>
> >>> Your patch does not provide persistent serial numbers, so a change to the 
> >>> QEMU
> >>> invocation could result in different serial numbers.  The guest will get
> >>> confused or perhaps refuse to boot.
> >>
> >>
> >> Yes, it introduce confusion, but in this way at least the serial number
> >> can be persistent across guest reboot. Traditionally ide disks use this
> >> way as well, such as QEMU_HARDISK_00001, I think guest should not use
> >> this in /etc/fstab.
> > 
> > If you don't want to set a persistent serial number, use another mechanism 
> > to
> > identify the disk.  For example, Linux has /dev/disk/by-path/ which 
> > identifies
> > virtio-blk PCI adapters, IDE, SCSI disks, etc.
> 
> >
> 
> > Does this work for your use case?
> 
> 
> I have tried this before, but after rebooting (kexec/kdump) the by-path
> link was not created. It might be udev bug anyway, I'm not sure though.

Yes, it's udev.  Check /lib/udev/rules.d/60-persistent-storage.rules.

Stefan