[libvirt-users] SCSI/IDE Devices in Guest

[Douglas Russell]

On 30 October 2012 20:49, Eric Blake <eblake at redhat.com> wrote:

> On 10/30/2012 08:46 AM, Douglas Russell wrote:
> > I'm experimenting with how to attach storage to a guest virtual machine
> (I
> > sent another message to th elist about that as it's slightly different)
> >
> > Looking in the XML files for my virtual machine, I see this:
> >
> > <disk type='file' device='disk'>
> >   <driver name='qemu' type='qcow2'/>
> >   <source file='/vm/myvm/tmpeuiVc9.qcow2'/>
> >   <target dev='hda' bus='ide'/>
> >   <address type='drive' controller='0' bus='0' unit='0'/>
> > </disk>
> >
> > However, /dev/hda is not a device in the guest. It is /dev/sda. It's not
> > really a problem in it's own right, but it's one of several connected
> > things which don't make sense. Is there some explanation for this?
>
> The <target dev='...'> designation is a hint, and there for you to track
> things in the host, but does not necessarily reflect the name that the
> guest will choose for the same device (there is NO way for the host to
> force what the guest names things).
>

Ok, that's fair enough, I was wondering how it would go about influencing
the guest.


> However, for SCSI devices in the guest, you probably want to use <disk
> type='block' device='lun'> rather than type='file' device='disk'.
>

You think I should change the above stanza or are you talking about the
second ? The above storage really is a file.


>
> [Caveat: I haven't used SCSI devices in my guests yet]
>
> >
> > I believe this is linked to some weirdness when I attach some extra
> storage
> > like so:
> >
> > virsh attach-disk myvm /dev/lvm-group1/mystorage sdb --driver qemu
> > --persistent
> >
> > This adds the following stanza:
> >
> > <disk type='block' device='disk'>
> >   <driver name='qemu' type='raw'/>
> >   <source dev='/dev/lvm-group1/mystorage'/>
> >   <target dev='sdb' bus='scsi'/>
> >   <address type='drive' controller='0' bus='0' unit='1'/>
> > </disk>
> >
> > Looks about right. In the guest though, it pushes /dev/sda (root and swap
> > are here on sda1 and sda2) upto /dev/sdb (sdb1 and sdb2). Somehow it
> still
> > boots correctly despite fstab referrencing /dev/sda1 and /dev/sda2 for /
> > and swap. The new /dev/sda is this storage I tried to add as sdb. What
> > could be causing this?
>
> Again, names in the XML don't have to reflect names chosen by the guest.
>  Beyond that, I don't have quite enough experience in this area of the
> code to give a definitive answer, but at least wanted to throw out an
> answer while you wait to see if anyone else also replies.
>

Ok, makes sense. I'm still wondering how it's able to start properly though
when the fstab is wrong though? It boots up and mounts /dev/sdb1 on /,
fstab still says /dev/sda1 is /.

I've also started a new thread as hotplug is also not working for some
reason.

Thanks.
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