Hi, since device UUIDs have been merged upstream, I think we could reheat the conversation about hotplug. This is the updated design doc. The implementation is ready but it is wiser to agree on the design first and make any changes if needed.
Looking forward to your feedback. Thanks in advance, dimara Signed-off-by: Dimitris Aragiorgis <[email protected]> --- Makefile.am | 1 + doc/design-draft.rst | 1 + doc/design-hotplug.rst | 220 ++++++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 222 insertions(+) create mode 100644 doc/design-hotplug.rst diff --git a/Makefile.am b/Makefile.am index 037cf53..155577d 100644 --- a/Makefile.am +++ b/Makefile.am @@ -407,6 +407,7 @@ docinput = \ doc/design-cpu-pinning.rst \ doc/design-device-uuid-name.rst \ doc/design-draft.rst \ + doc/design-hotplug.rst \ doc/design-htools-2.3.rst \ doc/design-http-server.rst \ doc/design-impexp2.rst \ diff --git a/doc/design-draft.rst b/doc/design-draft.rst index ccb2f93..3ec9c46 100644 --- a/doc/design-draft.rst +++ b/doc/design-draft.rst @@ -19,6 +19,7 @@ Design document drafts design-storagetypes.rst design-reason-trail.rst design-device-uuid-name.rst + design-hotplug.rst .. vim: set textwidth=72 : .. Local Variables: diff --git a/doc/design-hotplug.rst b/doc/design-hotplug.rst new file mode 100644 index 0000000..d34179f --- /dev/null +++ b/doc/design-hotplug.rst @@ -0,0 +1,220 @@ +======= +Hotplug +======= + +.. contents:: :depth: 4 + +This is a design document detailing the implementation of device +hotplugging in Ganeti. The logic used is hypervisor agnostic but still +the initial implementation will target the KVM hypervisor. The +implementation adds ``python-fdsend`` as a new dependency. + + +Current state and shortcomings +============================== + +Currently, Ganeti supports addition/removal/modification of devices +(NICs, Disks) but the actual modification takes place only after +rebooting the instance. To this end an instance cannot change network, +get a new disk etc. without a hard reboot. + +Until now, in case of KVM hypervisor, code does not name devices nor +places them in specific PCI slots. Devices are appended in the KVM +command and Ganeti lets KVM decide where to place them. This means that +there is a possibility a device that resides in PCI slot 5, after a +reboot (due to another device removal) to be moved to another PCI slot +and probably get renamed too (due to udev rules, etc.). + +In order migration to succeed, the process on the target node should be +started with exactly the same machine version, CPU architecture and PCI +configuration with the running process. During instance creation/startup +ganeti creates a KVM runtime file with all the necessary information to +generate the KVM command. This runtime file is used during instance +migration to start a new identical KVM process. The current format +includes the fixed part of the final KVM command a list of NICs', +and hvparams dict. It does not favor easy manipulations concerning +disks, because they are encapsulated in the fixed KVM command. + +Proposed changes +================ + +For the case of the KVM hypervisor, QEMU exposes 32 PCI slots to the +instance. Disks and NICs occupy some of these slots. Recent versions of +QEMU have introduced monitor commands that allow addition/removal of PCI +devices. Devices are referenced based on their name or position on the +virtual PCI bus. To be able to use these commands, we need to be able to +assign each device a unique name. For that we build on top of devices' +UUIDs. + +Finally, to keep track where each device is plugged into, we add the +``pci`` slot to Disk and NIC objects, but we save it only in runtime +files, since it is hypervisor specific info. + +We propose to make use of QEMU 1.0 monitor commands so that +modifications to devices take effect instantly without the need for hard +reboot. The only change exposed to the end-user will be the addition of +a ``--hotplug`` option to the ``gnt-instance modify`` command. + +Upon hotplugging the PCI configuration of an instance is changed. +Runtime files should be updated correspondingly. Currently this is +impossible in case of disk hotplug because disks are included in command +line entry of the runtime file, contrary to NICs that are correctly +treated separately. We change the format of runtime files, we remove +disks from the fixed KVM command and create new entry containing them +only. KVM options concerning disk are generated during +``_ExecuteKVMCommand()``, just like NICs. + +Design decisions +================ + +Which should be each device ID? Currently KVM does not support arbitrary +IDs for devices; supported are only names starting with a letter, max 32 +chars length, and only including '.' '_' special chars. To this end we +use a helper function that converts UUID to accepted device id. For the +sake of simplicity and readability we use the part of UUID until the +first dash and insert a "x" at the beginning of the string. + +Who decides where to hotplug each device? As long as this is a +hypervisor specific matter, there is no point for the master node to +decide such a thing. Master node just has to request noded to hotplug a +device. To this end, hypervisor specific code should parse the current +PCI configuration (i.e. ``info pci`` QEMU monitor command), find the first +available slot and hotplug the device. Having noded to decide where to +hotplug a device we ensure that no error will occur due to duplicate +slot assignment (if masterd keeps track of PCI reservations and noded +fails to return the PCI slot that the device was plugged into then next +hotplug will fail). + +Where should we keep track of devices' PCI slots? As already mentioned, +we must keep track of devices PCI slots to successfully migrate +instances. First option is to save this info to config data, which would +allow us to place each device at the same PCI slot after reboot. This +would require to make the hypervisor return the PCI slot chosen for each +device, and storing this information to config data. Additionally the +whole instance configuration should be returned with PCI slots filled +after instance start and each instance should keep track of current PCI +reservations. We decide not to go towards this direction in order to +keep it simple and do not add hypervisor specific info to configuration +data (``pci_reservations`` at instance level and ``pci`` at device +level). For the aforementioned reason, we decide to store this info only +in KVM runtime files. + +Where to place the devices upon instance startup? QEMU has by default 4 +pre-occupied PCI slots. So, hypervisor can use the remaining ones for +disks and NICs. Currently, PCI configuration is not preserved after +reboot. Each time an instance starts, KVM assigns PCI slots to devices +based on their ordering in Ganeti configuration, i.e. the second disk +will be placed after the first, the third NIC after the second, etc. +Since we decided that there is no need to keep track of devices PCI +slots, there is no need to change current functionality. + +How to deal with existing instances? Migration and hotplugging of +existing instances in a cluster after installing the version that +supports hotplug will not be supported until we run a specific script in +all nodes that adds a new entry for the block devices (e.g. []) in +existing runtime files or else until instances suffer a hard reboot. + + +Configuration changes +--------------------- + +The ``NIC`` and ``Disk`` objects get one extra slot: ``pci``. It refers to +PCI slot that the device gets plugged into. + +In order to be able to live migrate successfully, runtime files should +be updated every time a live modification (hotplug) takes place. To this +end we change the format of runtime files. The KVM options referring to +instance's disks are no longer recorded as part of the KVM command line. +Disks are treated separately, just as we treat NICs right now. We insert +and remove entries to reflect the current PCI configuration. + + +Backend changes +--------------- + +Introduce 4 new RPC calls: + +- hot_add_nic +- hot_del_nic +- hot_add_disk +- hot_del_disk + + +Hypervisor changes +------------------ + +We implement hotplug on top of the KVM hypervisor. We take advantage of +QEMU 1.0 monitor commands (``device_add``, ``device_del``, +``drive_add``, ``drive_del``, ``netdev_add``,`` netdev_del``). QEMU +refers to devices based on their id. We use ``uuid`` to name them +properly. If a device is about to be hotplugged we parse the output of +``info pci`` and find the occupied PCI slots. We choose the first +available and the whole device object is appended to the corresponding +entry in the runtime file. + +Concerning NIC handling, we build on the top of the existing logic +(first create a tap with _OpenTap() and then pass its file descriptor to +the KVM process). To this end we need to pass access rights to the +corresponding file descriptor over the monitor socket (UNIX domain +socket). The open file is passed as a socket-level control message +(SCM), using the ``fdsend`` python library. + + +User interface +-------------- + +The new ``--hotplug`` option to gnt-instance modify is introduced, which +forces live modifications. + + +Enabling hotplug +++++++++++++++++ + +Hotplug will be optional during gnt-instance modify. For existing +instances inside an old ganeti cluster we have two options after +upgrading to a version that supports hotplugging: + +1. Instances in the cluster should be hard rebooted or + +2. A specific script should change their runtime format before they can +support hotplugging of devices. + +If neither happens these instances will not support migration nor +hotplug. + + +NIC hotplug ++++++++++++ + +The user can add/modify/remove NICs either with hotplugging or not. If a +NIC is to be added a tap is created first and configured properly with +kvm-vif-bridge script. Then the instance gets a new network interface. +Since there is no QEMU monitor command to modify a NIC, we modify a NIC +by temporary removing the existing one and adding a new with the new +configuration. When removing a NIC the corresponding tap gets removed as +well. + +:: + + gnt-instance modify --net add --hotplug test + gnt-instance modify --net 1:mac=aa:00:00:55:44:33 --hotplug test + gnt-instance modify --net 1:remove --hotplug test + + +Disk hotplug +++++++++++++ + +The user can add and remove disks with hotplugging or not. QEMU monitor +supports resizing of disks, however the initial implementation will +support only disk addition/deletion. + +:: + + gnt-instance modify --disk add:size=1G --hotplug test + gnt-instance modify --disk 1:remove --hotplug test + +.. vim: set textwidth=72 : +.. Local Variables: +.. mode: rst +.. fill-column: 72 +.. End: -- 1.7.10.4
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