5 .. contents:: :depth: 4
7 This is a design document detailing the implementation of device
8 hotplugging in Ganeti. The logic used is hypervisor agnostic but still
9 the initial implementation will target the KVM hypervisor. The
10 implementation adds ``python-fdsend`` as a new dependency. In case
11 it is not installed hotplug will not be possible and the user will
12 be notified with a warning.
15 Current state and shortcomings
16 ==============================
18 Currently, Ganeti supports addition/removal/modification of devices
19 (NICs, Disks) but the actual modification takes place only after
20 rebooting the instance. To this end an instance cannot change network,
21 get a new disk etc. without a hard reboot.
23 Until now, in case of KVM hypervisor, code does not name devices nor
24 places them in specific PCI slots. Devices are appended in the KVM
25 command and Ganeti lets KVM decide where to place them. This means that
26 there is a possibility a device that resides in PCI slot 5, after a
27 reboot (due to another device removal) to be moved to another PCI slot
28 and probably get renamed too (due to udev rules, etc.).
30 In order for a migration to succeed, the process on the target node
31 should be started with exactly the same machine version, CPU
32 architecture and PCI configuration with the running process. During
33 instance creation/startup ganeti creates a KVM runtime file with all the
34 necessary information to generate the KVM command. This runtime file is
35 used during instance migration to start a new identical KVM process. The
36 current format includes the fixed part of the final KVM command, a list
37 of NICs', and hvparams dict. It does not favor easy manipulations
38 concerning disks, because they are encapsulated in the fixed KVM
45 For the case of the KVM hypervisor, QEMU exposes 32 PCI slots to the
46 instance. Disks and NICs occupy some of these slots. Recent versions of
47 QEMU have introduced monitor commands that allow addition/removal of PCI
48 devices. Devices are referenced based on their name or position on the
49 virtual PCI bus. To be able to use these commands, we need to be able to
50 assign each device a unique name.
52 To keep track where each device is plugged into, we add the
53 ``pci`` slot to Disk and NIC objects, but we save it only in runtime
54 files, since it is hypervisor specific info. This is added for easy
55 object manipulation and is ensured not to be written back to the config.
57 We propose to make use of QEMU 1.0 monitor commands so that
58 modifications to devices take effect instantly without the need for hard
59 reboot. The only change exposed to the end-user will be the addition of
60 a ``--hotplug`` option to the ``gnt-instance modify`` command.
62 Upon hotplugging the PCI configuration of an instance is changed.
63 Runtime files should be updated correspondingly. Currently this is
64 impossible in case of disk hotplug because disks are included in command
65 line entry of the runtime file, contrary to NICs that are correctly
66 treated separately. We change the format of runtime files, we remove
67 disks from the fixed KVM command and create new entry containing them
68 only. KVM options concerning disk are generated during
69 ``_ExecuteKVMCommand()``, just like NICs.
74 Which should be each device ID? Currently KVM does not support arbitrary
75 IDs for devices; supported are only names starting with a letter, max 32
76 chars length, and only including '.' '_' '-' special chars.
77 For debugging purposes and in order to be more informative, device will be
78 named after: <device type>-<part of uuid>-pci-<slot>.
80 Who decides where to hotplug each device? As long as this is a
81 hypervisor specific matter, there is no point for the master node to
82 decide such a thing. Master node just has to request noded to hotplug a
83 device. To this end, hypervisor specific code should parse the current
84 PCI configuration (i.e. ``info pci`` QEMU monitor command), find the first
85 available slot and hotplug the device. Having noded to decide where to
86 hotplug a device we ensure that no error will occur due to duplicate
87 slot assignment (if masterd keeps track of PCI reservations and noded
88 fails to return the PCI slot that the device was plugged into then next
91 Where should we keep track of devices' PCI slots? As already mentioned,
92 we must keep track of devices PCI slots to successfully migrate
93 instances. First option is to save this info to config data, which would
94 allow us to place each device at the same PCI slot after reboot. This
95 would require to make the hypervisor return the PCI slot chosen for each
96 device, and storing this information to config data. Additionally the
97 whole instance configuration should be returned with PCI slots filled
98 after instance start and each instance should keep track of current PCI
99 reservations. We decide not to go towards this direction in order to
100 keep it simple and do not add hypervisor specific info to configuration
101 data (``pci_reservations`` at instance level and ``pci`` at device
102 level). For the aforementioned reason, we decide to store this info only
103 in KVM runtime files.
105 Where to place the devices upon instance startup? QEMU has by default 4
106 pre-occupied PCI slots. So, hypervisor can use the remaining ones for
107 disks and NICs. Currently, PCI configuration is not preserved after
108 reboot. Each time an instance starts, KVM assigns PCI slots to devices
109 based on their ordering in Ganeti configuration, i.e. the second disk
110 will be placed after the first, the third NIC after the second, etc.
111 Since we decided that there is no need to keep track of devices PCI
112 slots, there is no need to change current functionality.
114 How to deal with existing instances? Hotplug depends on runtime file
115 manipulation. It stores there pci info and every device the kvm process is
116 currently using. Existing files have no pci info in devices and have block
117 devices encapsulated inside kvm_cmd entry. Thus hotplugging of existing devices
118 will not be possible. Still migration and hotplugging of new devices will
119 succeed. The workaround will happen upon loading kvm runtime: if we detect old
120 style format we will add an empty list for block devices and upon saving kvm
121 runtime we will include this empty list as well. Switching entirely to new
122 format will happen upon instance reboot.
125 Configuration changes
126 ---------------------
128 The ``NIC`` and ``Disk`` objects get one extra slot: ``pci``. It refers to
129 PCI slot that the device gets plugged into.
131 In order to be able to live migrate successfully, runtime files should
132 be updated every time a live modification (hotplug) takes place. To this
133 end we change the format of runtime files. The KVM options referring to
134 instance's disks are no longer recorded as part of the KVM command line.
135 Disks are treated separately, just as we treat NICs right now. We insert
136 and remove entries to reflect the current PCI configuration.
142 Introduce one new RPC call:
144 - hotplug_device(DEVICE_TYPE, ACTION, device, ...)
146 where DEVICE_TYPE can be either NIC or Disk, and ACTION either REMOVE or ADD.
151 We implement hotplug on top of the KVM hypervisor. We take advantage of
152 QEMU 1.0 monitor commands (``device_add``, ``device_del``,
153 ``drive_add``, ``drive_del``, ``netdev_add``,`` netdev_del``). QEMU
154 refers to devices based on their id. We use ``uuid`` to name them
155 properly. If a device is about to be hotplugged we parse the output of
156 ``info pci`` and find the occupied PCI slots. We choose the first
157 available and the whole device object is appended to the corresponding
158 entry in the runtime file.
160 Concerning NIC handling, we build on the top of the existing logic
161 (first create a tap with _OpenTap() and then pass its file descriptor to
162 the KVM process). To this end we need to pass access rights to the
163 corresponding file descriptor over the monitor socket (UNIX domain
164 socket). The open file is passed as a socket-level control message
165 (SCM), using the ``fdsend`` python library.
171 The new ``--hotplug`` option to gnt-instance modify is introduced, which
172 forces live modifications.
178 Hotplug will be optional during gnt-instance modify. For existing
179 instance, after installing a version that supports hotplugging we
180 have the restriction that hotplug will not be supported for existing
181 devices. The reason is that old runtime files lack of:
183 1. Device pci configuration info.
185 2. Separate block device entry.
187 Hotplug will be supported only for KVM in the first implementation. For
188 all other hypervisors, backend will raise an Exception case hotplug is
195 The user can add/modify/remove NICs either with hotplugging or not. If a
196 NIC is to be added a tap is created first and configured properly with
197 kvm-vif-bridge script. Then the instance gets a new network interface.
198 Since there is no QEMU monitor command to modify a NIC, we modify a NIC
199 by temporary removing the existing one and adding a new with the new
200 configuration. When removing a NIC the corresponding tap gets removed as
205 gnt-instance modify --net add --hotplug test
206 gnt-instance modify --net 1:mac=aa:00:00:55:44:33 --hotplug test
207 gnt-instance modify --net 1:remove --hotplug test
213 The user can add and remove disks with hotplugging or not. QEMU monitor
214 supports resizing of disks, however the initial implementation will
215 support only disk addition/deletion.
219 gnt-instance modify --disk add:size=1G --hotplug test
220 gnt-instance modify --net 1:remove --hotplug test
223 Dealing with chroot and uid pool
224 --------------------------------
226 The design so far covers all issues that arise without addressing the
227 case where the kvm process will not run with root privileges.
230 - in case of chroot, the kvm process cannot see the newly created device
232 - in case of uid pool security model, the kvm process is not allowed
235 For NIC hotplug we address this problem by using the ``getfd`` monitor
236 command and passing the file descriptor to the kvm process over the
237 monitor socket using SCM_RIGHTS. For disk hotplug and in case of uid
238 pool we can let the hypervisor code temporarily ``chown()`` the device
239 before the actual hotplug. Still this is insufficient in case of chroot.
240 In this case, we need to ``mknod()`` the device inside the chroot. Both
241 workarounds can be avoided, if we make use of the ``add-fd`` qemu
242 monitor command, that was introduced in version 1.3. This command is the
243 equivalent of NICs' `get-fd`` for disks and will allow disk hotplug in
244 every case. So, if the qemu monitor does not support the ``add-fd``
245 command, we will not allow disk hotplug for chroot and uid security
246 model and notify the user with the corresponding warning.
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projects / ganeti-local / blob