+lists Hi.
Comments inline. * Guido Trotter <[email protected]> [2013-05-21 15:08:56 +0200]: > On Sat, May 18, 2013 at 3:25 AM, Dimitris Aragiorgis <[email protected]>wrote: > > > 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. > > + > > > > What is the purpose of a "pci" slot to disk and nic *objects* if these are > never saved in the config, but only kept in KVM specific runtime files? > In order to easily create NIC and Disk objects from serialized (json) data. > +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. > > + > > > > How do we guarantee that no two devices will have the same name? We should > double check this, to be sure there are no conflicts. > > Well we actually don't. Trimming system's generated uuid rises our chances for duplicate IDs. If there is a way to generate a uuid of max 32 chars starting with a letter based on the uuids of type /proc/sys/kernel/random/uuid then it would be great. Until we find one, we could disable hotplug for devices that have the uuid prefix. > > > +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). > > + > > > > Ack. > > > > +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. > > + > > > > Ack. qq: do we need to store them at all, or could we get them from kvm on > the source machine, and use them on the destination machine, upon migration? > > Well the pci_resevations is actually needed upon instance startup as well. Let's say we have an instance with pci slots 5, 6, 8, 10 reserved and the corresponding device objects with their pci slot up-to-date. We bring this machine down. We add another device. Upon instance startup how do we know where to place that device? QEMU monitor cannot be asked because instance is down. Before hotplugging devices were placed automatically by qemu. First the disks then the nics. This resulted to pci renumbering. > > +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. > > > > Ouch. Can we support this better? :) If we go for "read current values from > the qemu monitor" rather than "store this info outside" we get this > magically working, no? > > Got lost :) What is "this"? "read current values from qemu monitor" works only if instance is running. Look previous comment. Existing instances do not have a pci configuration. Still they have the old runtime file format (that does not treat disks as a different entry). This is the reason migration will fail. > > +Configuration changes > > +--------------------- > > + > > +The ``NIC`` and ``Disk`` objects get one extra slot: ``pci``. It refers to > > +PCI slot that the device gets plugged into. > > + > > > > Why? You seem not to be using this anyway. > Explained before. > > > +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. > > > > Ack. > > > > + > > + > > +Backend changes > > +--------------- > > + > > +Introduce 4 new RPC calls: > > + > > +- hot_add_nic > > +- hot_del_nic > > +- hot_add_disk > > +- hot_del_disk > > + > > > > Does it make sense to have one call for device type? Could we not use one > call for both? Well I implemented it so that it is more straightforward. We could have hotplug_device(device, action) and act depending the device type (NIC, Disk) and the action (ADD, REMOVE) > As for del, do we have any concerns? (this can easily break an instance I > suppose, am I wrong? can it fail if the device is in use? How do we handle > these cases?) > Unplugging a device has concerns but there are cases we want this to happen. disable an instance networking, renumber an instance IP, remove a disk (like unplugging an USB), etc. With existing implementation when we remove a disk from a running instance it fails because it is used by QEMU. With hotplug we hot_del_disk then _ShutdownInstanceDisks() then _AnnotateDiskParams() and then blockdev_remove(). > > > + > > +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. > > + > > > > Is there any way to have this as a recommends but not depends (aka: > hotplugging won't work if this is not present, but everything else will)? > Sure. > > > + > > +User interface > > +-------------- > > + > > +The new ``--hotplug`` option to gnt-instance modify is introduced, which > > +forces live modifications. > > + > > > > What happens if they are unsupported (eg. other hypervisors), does the > change fail? In cmdlib we get instance's hypervisor before hotplugging devices. There we could see if the corresponding hypervisor supports it or not. > Should there be a way to use hotplug by default? (now, or later perhaps)? > Yes. Switch --hotplug option to --no-hotplug. :) > > > + > > +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. > > + > > > > not supporting *migration*: definitely bad. > > Agree. The old format of the runtime files to blame.. :) > > + > > +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 > > + > > > > s/test/instance.example.com/ (so it's clear what it is) > Also is it safe to "just" add and remove a nic from an instance? This is > going to break things, I suppose. > > Not really. Just the instance will see an interface to disappear. If this is the primary interface then it will lose internet connection (if any). > > + > > +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. > > + > > > > It would definitely be nice to support resizing too, but ok, it can go in > later. > > AGREE! But please later.... > > +:: > > + > > + gnt-instance modify --disk add:size=1G --hotplug test > > + gnt-instance modify --disk 1:remove --hotplug test > > + > > > > Thanks, > I'll try to implement something of the aforementioned and rebase it to current master, you know, the one with NO cmdlib.py! Thanks for the feedback, dimara
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