Currently, tasks and variants in everygreen.yml both define the storageEngine argument to resmoke.py. The tasks define either mmapv1 or wiredTiger and a variant that uses a different storage engine would override that.

The override is implicitly enabled by having the resmoke arguments defined for a variant ("test_flags") be passed in later than the resmoke arguments for the task ("resmoke_args").

The side-effect of this ordering is subtle and could cause breakages when we we make changes to how resmoke is invoked.

A better way would be to make the storage engine a variable. E.g.

- <<: *task_template
  name: jsCore
  commands:
  - func: "do setup"
  - func: "run tests"
    vars:
      resmoke_args: --suites=core --storageEngine=mmapv1
      run_multiple_jobs: true

becomes:

- <<: *task_template
  name: jsCore_mmapv1
  commands:
  - func: "do setup"
  - func: "run tests"
    vars:
      resmoke_args: --suites=core
      run_multiple_jobs: true
      task_storage_engine: 'mmapv1'

Variants can similarly define a variant_storage_engine which can then explicitly override the storage engine of the task when constructing resmoke arguments in run_tests (or define task definitions without storage engines)