This patch adds a design document detailing the implementation of disks as new top-level citizens in the config file (just like instances, nodes etc).
The process is divided in four steps, and currently only the first one is being described in depth. Signed-off-by: Ilias Tsitsimpis <[email protected]> --- Hello team, I'm sending the revised design doc, after incorporating all your comments from the corresponding thread. Thanks a lot, Ilias doc/design-disks.rst | 166 +++++++++++++++++++++++++++++++++++++++++++++++++++ doc/index.rst | 1 + 2 files changed, 167 insertions(+) create mode 100644 doc/design-disks.rst diff --git a/doc/design-disks.rst b/doc/design-disks.rst new file mode 100644 index 0000000..61900cb --- /dev/null +++ b/doc/design-disks.rst @@ -0,0 +1,166 @@ +===== +Disks +===== + +.. contents:: :depth: 4 + +This is a design document detailing the implementation of disks as a new +top-level citizen in the config file (just like instances, nodes etc). + + +Current state and shortcomings +============================== + +Currently, Disks are stored in Ganeti's config file as a list +(container) of Disk objects under the Instance in which they belong. +This implementation imposes a number of limitations: + +* A Disk object cannot live outside an Instance. This means that one + cannot detach a disk from an instance (without destroying the disk) + and then reattach it (to the same or even to a different instance). + +* Disks are not taggable objects, as only top-level citizens of the + config file can be made taggable. Having taggable disks will allow for + further customizations. + + +Proposed changes +================ + +The implementation is going to be split in four parts: + +* Make disks a top-level citizen in config file. The Instance object + will no longer contain a list of Disk objects, but a list of disk + UUIDs. + +* Add locks for Disk objects and make them taggable. + +* Allow to attach/detach an existing disk to/from an instance. + +* Allow creation/modification/deletion of disks that are not attached to + any instance (requires new LUs for disks). + + +Design decisions +================ + +Disks as config top-level citizens +---------------------------------- + +The first patch-series is going to add a new top-level citizen in the +config object (namely ``disks``) and separate the disk objects from the +instances. In doing so there are a number of problems that we have to +overcome: + +* How the Disk object will be represented in the config file and how it + is going to be connected with the instance it belongs to. + +* How the existing code will get the disks belonging to an instance. + +* What it means for a disk to be attached/detached to/from an instance. + +* How disks are going to be created/deleted, attached/detached using + the existing code. + + +Disk representation +~~~~~~~~~~~~~~~~~~~ + +The ``Disk`` object gets two extra slots, ``_TIMESTAMPS`` and +``serial_no``. + +The ``Instance`` object will no longer contain the list of disk objects +that are attached to it. Instead, an Instance object will refer to its +disks using their UUIDs. Since the order in which the disks are attached +to an instance is important we are going to have a list of disk UUIDs +under the Instance object which will denote the disks attached to the +instance and their order at the same time. So the Instance's ``disks`` +slot is going to be a list of disk UUIDs. The `Disk` object is not going +to have a slot pointing to the `Instance` in which it belongs since this +is redundant. + + +Get instance's disks +~~~~~~~~~~~~~~~~~~~~ + +A new function ``GetInstanceDisks`` will be added to the config that given an +instance will return a list of Disk objects with the disks attached to this +instance. This list will be exactly the same as 'instance.disks' was before. +Everywhere in the code we are going to replace the 'instance.disks' (which from +now one will contain a list of disk UUIDs) with the function +``GetInstanceDisks``. + +Since disks will not be part of the `Instance` object any more, 'all_nodes' and +'secondary_nodes' can not be `Instance`'s properties. Instead we will use the +functions ``GetInstanceNodes`` and ``GetInstanceSecondaryNodes`` from the +config to compute these values. + + +Configuration changes +~~~~~~~~~~~~~~~~~~~~~ + +The ``ConfigData`` object gets one extra slot: ``disks``. Also there +will be two new functions, ``AddDisk`` and ``RemoveDisk`` that will +create/remove a disk objects from the config. + +The ``VerifyConfig`` function will be changed so it can check that there +are no dangling pointers from instances to disks (i.e. an instance +points to a disk that doesn't exist in the config). + +The 'upgrade' operation for the config should check if disks are top level +citizens and if not it has to extract the disk objects from the instances and +replace them with their uuids. In case of the 'downgrade' operation (where +disks will be made again part of the `Instance` object) all disks that are not +attached to any instance at all will be ignored (removed from config). + + +Apply Disk modifications +~~~~~~~~~~~~~~~~~~~~~~~~ + +There are four operations that can be performed to a `Disk` object: + +* Create a new `Disk` object and save it to the config. + +* Remove an existing `Disk` object from the config. + +* Attach an existing `Disk` to an existing `Instance`. + +* Detach an existing `Disk` from an existing `Instance`. + +The first two operations will be performed using the config functions +``AddDisk`` and ``RemoveDisk`` respectively where the last two operations +will be performed using the functions ``AttachInstDisk`` and +``DetachInstDisk``. + +Since Ganeti doesn't allow for a `Disk` object to not be attached anywhere (for +now) we will create two wrapper functions (namely ``AddInstDisk`` and +``RemoveInstDisk``) which will add and attach a disk at the same time +(respectively detach and remove a disk). + +In addition since Ganeti doesn't allow for a `Disk` object to be attached to +more than one `Instance` at once, when attaching a disk to an instance we have +to make sure that the disk is not attached anywhere else. + + +Backend changes +~~~~~~~~~~~~~~~ + +The backend needs access to the disks of an `Instance` but doesn't have access to +the `GetInstanceDisks` function from the config file. Thus we will create a new +`Instance` slot (namely ``disks_info``) that will get annotated (during RPC) +with the instance's disk objects. So in the backend we will only have to +replace the ``disks`` slot with ``disks_info``. + + +.. TODO: Locks for Disk objects + +.. TODO: Attach/Detach disks + +.. TODO: LUs for disks + + +.. vim: set textwidth=72 : +.. Local Variables: +.. mode: rst +.. fill-column: 72 +.. End: diff --git a/doc/index.rst b/doc/index.rst index ab2cccf..419d560 100644 --- a/doc/index.rst +++ b/doc/index.rst @@ -97,6 +97,7 @@ Draft designs design-cmdlib-unittests.rst design-cpu-pinning.rst design-device-uuid-name.rst + design-disks.rst design-file-based-storage.rst design-hroller.rst design-hotplug.rst -- 1.9.0
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