Public bug reported: The q35 machine type support native pcie instead of legacy ahci based pci hotplug. This has several advantages and one majour disadvantage with the new pcie approch you need to pre allocate the pcie slot so that they will be available for use with hotplug if needed.
to support this a new num_pcie_ports config option was added to the libvirt section of the nova.conf https://docs.openstack.org/nova/latest/configuration/config.html#libvirt.num_pcie_ports the default value of this config is 0 which mean we use libvirts default. libvirts default is to allocate 1 free pcie port as a result by default you cannot attach more then 1 device without hard rebooting the vm. previously when using the pc machine type with the i440fx chipset it was possible to attach multiple interfaces or volumes. as a result the end user behavior has changed as observed by the failrure in tempest.api.compute.servers.test_attach_interfaces.AttachInterfacesTestJSON.test_create_list_show_delete_interfaces_by_network_port with the default setting and q35 enabled as reported in this downstream bug https://bugzilla.redhat.com/show_bug.cgi?id=1716356 to fix this the suggestion is to set the max value and default value of the num_pcie_ports config option to 32 based on some minimal local testing the memory usage of this change is ~0.4MB per port or ~12.5 mb per vm in addtion qemu overhead. this is based on testing done with libvirt directly with memroy preallocation enabeld for a 2G guest with the pc machinetype and i440fx chipset total memroy of 2036 MB was observed, q35 4 ports (the default value that will be calulated by libvirt for the default devices) increesed this to 2056 MB and q35 32 ports to 2066 MB as such this i a minimal overhead increase which can still be controlled by setting the config to a lower value explicitly. ** Affects: nova Importance: Low Assignee: Kashyap Chamarthy (kashyapc) Status: In Progress ** Tags: libvirt -- You received this bug notification because you are a member of Yahoo! Engineering Team, which is subscribed to OpenStack Compute (nova). https://bugs.launchpad.net/bugs/1831886 Title: default pcie hotplug behavior changes when using q35 Status in OpenStack Compute (nova): In Progress Bug description: The q35 machine type support native pcie instead of legacy ahci based pci hotplug. This has several advantages and one majour disadvantage with the new pcie approch you need to pre allocate the pcie slot so that they will be available for use with hotplug if needed. to support this a new num_pcie_ports config option was added to the libvirt section of the nova.conf https://docs.openstack.org/nova/latest/configuration/config.html#libvirt.num_pcie_ports the default value of this config is 0 which mean we use libvirts default. libvirts default is to allocate 1 free pcie port as a result by default you cannot attach more then 1 device without hard rebooting the vm. previously when using the pc machine type with the i440fx chipset it was possible to attach multiple interfaces or volumes. as a result the end user behavior has changed as observed by the failrure in tempest.api.compute.servers.test_attach_interfaces.AttachInterfacesTestJSON.test_create_list_show_delete_interfaces_by_network_port with the default setting and q35 enabled as reported in this downstream bug https://bugzilla.redhat.com/show_bug.cgi?id=1716356 to fix this the suggestion is to set the max value and default value of the num_pcie_ports config option to 32 based on some minimal local testing the memory usage of this change is ~0.4MB per port or ~12.5 mb per vm in addtion qemu overhead. this is based on testing done with libvirt directly with memroy preallocation enabeld for a 2G guest with the pc machinetype and i440fx chipset total memroy of 2036 MB was observed, q35 4 ports (the default value that will be calulated by libvirt for the default devices) increesed this to 2056 MB and q35 32 ports to 2066 MB as such this i a minimal overhead increase which can still be controlled by setting the config to a lower value explicitly. To manage notifications about this bug go to: https://bugs.launchpad.net/nova/+bug/1831886/+subscriptions -- Mailing list: https://launchpad.net/~yahoo-eng-team Post to : [email protected] Unsubscribe : https://launchpad.net/~yahoo-eng-team More help : https://help.launchpad.net/ListHelp
