Submitting jobs from logical units

Created:2011-Mar-23 Status:Implemented Ganeti-Version:2.5.0

Contents

• Submitting jobs from logical units
  • Current state and shortcomings
  • Proposed changes
  • Other discussed solutions

This is a design document about the innards of Ganeti’s job processing. Readers are advised to study previous design documents on the topic:

• Original job queue
• Job priorities

Current state and shortcomings

Some Ganeti operations want to execute as many operations in parallel as possible. Examples are evacuating or failing over a node (gnt-node evacuate/gnt-node failover). Without changing large parts of the code, e.g. the RPC layer, to be asynchronous, or using threads inside a logical unit, only a single operation can be executed at a time per job.

Currently clients work around this limitation by retrieving the list of desired targets and then re-submitting a number of jobs. This requires logic to be kept in the client, in some cases leading to duplication (e.g. CLI and RAPI).

Proposed changes

The job queue lock is guaranteed to be released while executing an opcode/logical unit. This means an opcode can talk to the job queue and submit more jobs. It then receives the job IDs, like any job submitter using the LUXI interface would. These job IDs are returned to the client, who then will then proceed to wait for the jobs to finish.

Technically, the job queue already passes a number of callbacks to the opcode processor. These are used for giving user feedback, notifying the job queue of an opcode having gotten its locks, and checking whether the opcode has been cancelled. A new callback function is added to submit jobs. Its signature and result will be equivalent to the job queue’s existing SubmitManyJobs function.

Logical units can submit jobs by returning an instance of a special container class with a list of jobs, each of which is a list of opcodes (e.g. [[op1, op2], [op3]]). The opcode processor will recognize instances of the special class when used a return value and will submit the contained jobs. The submission status and job IDs returned by the submission callback are used as the opcode’s result. It should be encapsulated in a dictionary allowing for future extensions.

Example:

{
  "jobs": [
    (True, "8149"),
    (True, "21019"),
    (False, "Submission failed"),
    (True, "31594"),
    ],
}

Job submissions can fail for variety of reasons, e.g. a full or drained job queue. Lists of jobs can not be submitted atomically, meaning some might fail while others succeed. The client is responsible for handling such cases.

Other discussed solutions

Instead of requiring the client to wait for the returned jobs, another idea was to do so from within the submitting opcode in the master daemon. While technically possible, doing so would have two major drawbacks:

• Opcodes waiting for other jobs to finish block one job queue worker thread
• All locks must be released before starting the waiting process, failure to do so can lead to deadlocks

Instead of returning the job IDs as part of the normal opcode result, introducing a new opcode field, e.g. op_jobids, was discussed and dismissed. A new field would touch many areas and possibly break some assumptions. There were also questions about the semantics.