[libvirt] pci bus slot in libvirt

[Daniel P. Berrange]

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.

> 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
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