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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.)

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1 Answer

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.

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