Take the 2-minute tour ×
Server Fault is a question and answer site for system and network administrators. It's 100% free, no registration required.

Oh RHEL 5.5, there are 31 ACPI PCI hotplug slots by default:

acpiphp: Slot [1] registered
acpiphp: Slot [31] registered

Is there a way to increase this number? I haven't been able to find an argument to supply to modprobe, or a sysctl knob to tweak, but I know there must be ways to get more slots based on some Google sleuthing.

(For the curious, this is just preliminary experimentation to see how many virtual disks I can hot-add to a running KVM guest.)

share|improve this question

1 Answer 1

I suppose I could have just read the docs...

Para-virtualized devices

Para-virtualized devices, which use the virtio drivers, are PCI devices. Presently, guests are limited to a maximum of 32 PCI devices. Some PCI devices are critical for the guest to run and these devices cannot be removed. The default, required devices are:

  • the host bridge,
  • the ISA bridge and usb bridge (The usb and isa bridges are the same device),
  • the graphics card (using either the Cirrus or qxl driver), and
  • the memory balloon device.

Out of the 32 available PCI devices for a guest 4 are not removable. This means there are only 28 PCI slots available for additional devices per guest. Every para-virtualized network or block device uses one slot. Each guest can use up to 28 additional devices made up of any combination of para-virtualized network, para-virtualized disk devices, or other PCI devices using VT-d.

Still, if anyone knows how to theoretically tweak the number of slots, I'll accept your answer.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.