Re: [libvirt] pci bus slot in libvirt

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 -- |: 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 :|