[libvirt] pci bus slot in libvirt

[Osier Yang]

On 2012年06月27日 05:25, Fong Vang wrote:
> On Tue, Jun 26, 2012 at 2:00 PM, Fong Vang<sudoyang at gmail.com>  wrote:
>> Very informational.  Thank you very much.
>>
>> On Tue, Jun 26, 2012 at 1:51 PM, Daniel P. Berrange<berrange at redhat.com>  wrote:
>>> On Tue, Jun 26, 2012 at 01:09:24PM -0700, Fong Vang wrote:
>>>> What's the best way to configure multiple disks in libvirt?  How many
>>>> slots can you have?  This is from the xml dump:
>>>>
>>>> In some of my configurations, I have this:
>>>>
>>>>      <disk type='block' device='disk'>
>>>>        <driver name='qemu' type='raw' cache='none' io='native'/>
>>>>        <source dev='/dev/sdb1'/>
>>>>        <target dev='vdb' bus='virtio'/>
>>>>        <alias name='virtio-disk1'/>
>>>>        <address type='pci' domain='0x0000' bus='0x00' slot='0x03'
>>>> function='0x0'/>
>>>>      </disk>
>>>
>>> With the traditional VirtIO-Block device, the disk controller only
>>> accepts a single drive, so each<disk>  you add consumes 1 PCI slot.
>>> You have a max of 31 PCI slots, of which at most 28 are usable for
>>> (PIIX consumes 1, VGA device consumes 1 and Balloon driver consumes
>>> 1). Those 28 must be split across everything else, so assuming no
>>> NICs or other devices, you have create 28 virtio-block disks.
>
> One last question on this.  Are there some slots that are reserved?

slot 0, 1, 2 are reserved. You will fail to start the guest anyway
if configure the devices using those 3 slot number. :-)


> In my setup, there could only be multiple disks and NICs.  I'm trying
> to figure out the best way to structure the different components with
> respect to slot numbering.
>
>>>
>>>> while others are created as such:
>>>>
>>>>    <devices>
>>>>      <emulator>/usr/bin/qemu-kvm</emulator>
>>>>      <disk type='file' device='disk'>
>>>>        <driver name='qemu' type='qcow2'/>
>>>>        <source file='/data2/VMs/libvirt/images/win2k8-1.qcow2'/>
>>>>        <target dev='hda' bus='ide'/>
>>>>        <alias name='ide0-0-0'/>
>>>>        <address type='drive' controller='0' bus='0' target='0' unit='0'/>
>>>>      </disk>
>>>
>>> This is an IDE disk, where the IDE controller consumes the PCI slot,
>>> and can support upto 4 drives (hda, hdb, hdc and hdd). So for 1 PCI
>>> slot, you get 4 drives, which isn't great.
>>>
>>>> Which way is better?  The top one defines the bus and slot while the
>>>> bottom defines the controller, bus, target, and unit.
>>>
>>> You won't typically choose between IDE and Virtio-Block based on the
>>> number of disks. You'll simply always want virtio-block for performance
>>> reasons. So there's no real choice of config here.
>>>
>>> With the latest libvirt, guest kernel and QEMU, there is a new device
>>> type known as Virtio-SCSI. In this the Virtio-SCSI controller consumes
>>> 1 PCI device and can handle many many many disks. This is the best
>>> option for the future for people who need lots of disks.
>>>
>>>> BTW, how many slots can we define per bus?
>>>
>>> With virtio-block, currently 28.
>>>
>>> If you are willing todo without hotplug support, you can turn on
>>> multi-function and increase the number of possible virtio-block
>>> devices x7 ie  ~196
>>>
>>> In the future we'll also have support for PCI bridges, and multiple
>>> PCI root complexes, which will increase max # of PCI slots even more.
>>>
>>> But virtio-SCSI really is the best way in the future.
>>>
>>>
>>> Daniel
>>> --
>>> |: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
>>> |: http://libvirt.org              -o-             http://virt-manager.org :|
>>> |: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
>>> |: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|
>
> --
> libvir-list mailing list
> libvir-list at redhat.com
> https://www.redhat.com/mailman/listinfo/libvir-list