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# Ganeti Bulk Create

Contents

- Ganeti Bulk Create
  - Current state and shortcomings
    - Proposed changes
    - Problems
  - Implementation
    - IAllocator
    - OpInstanceBulkAdd

## Current state and shortcomings

Creation of instances happens a lot. A fair load is done by just creating instances and due to bad allocation shifting them around later again. Additionally, if you turn up a new cluster you already know a bunch of instances, which need to exists on the cluster. Doing this one-by-one is not only cumbersome but might also fail, due to lack of resources or lead to badly balanced clusters.

Since the early Ganeti 2.0 alpha version there is a `gnt-instance batch-create` command to allocate a bunch of instances based on a json file. This feature, however, doesn’t take any advantages of iallocator and submits jobs in a serialized manner.

### Proposed changes

To overcome this shortcoming we would extend the current iallocator interface to allow bulk requests. On the Ganeti side, a new opcode is introduced to handle the bulk creation and returning the resulting placement from the IAllocator.

### Problems

Due to the design of chained jobs, we can guarantee, that with the state at which the `multi-alloc` opcode is run, all of the instances will fit (or all won’t). But we can’t guarantee that once the instance creation requests were submit, no other jobs have sneaked in between. This might still lead to failing jobs because the resources have changed in the meantime.

## Implementation

### IAllocator

A new additional `type` will be added called `multi-allocate` to distinguish between normal and bulk operation. For the bulk operation the `request` will be a finite list of request dicts.

If `multi-allocate` is declared, `request` must exist and is a list of `request` dicts as described in Operation specific input. The `result` then is a list of instance name and node placements in the order of the `request` field.

In addition, the old `allocate` request type will be deprecated and at latest in Ganeti 2.8 incooperated into this new request. Current code will need slight adaption to work with the new request. This needs careful testing.

### OpInstanceBulkAdd

We add a new opcode `OpInstanceBulkAdd`. It receives a list of `OpInstanceCreate` on the `instances` field. This is done to make sure, that these two loosely coupled opcodes do not get out of sync. On the RAPI side, however, this just is a list of instance create definitions. And the client is adapted accordingly.

The opcode itself does some sanity checks on the instance creation opcodes which includes:

- `mode` is not set
- `pnode` and `snodes` is not set
- `iallocator` is not set

Any of the above error will be aborted with `OpPrereqError`. Once the list has been verified it is handed to the `iallocator` as described in IAllocator. Upon success we then return the result of the IAllocator call.

At this point the current instance allocation would work with the resources available on the cluster as perceived upon `OpInstanceBulkAdd` invocation. However, there might be corner cases where this is not true as described in Problems.

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