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Re: [libvirt] pci bus slot in libvirt



Very informational.  Thank you very much.

On Tue, Jun 26, 2012 at 1:51 PM, Daniel P. Berrange <berrange 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.
>
>> 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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