Synnefo

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from __future__ import division

def allocate(backends, vm):

    if len(backends) == 1:

        return backends.keys()[0]

    # Filter those that can not host the vm

    capable_backends = dict((k, v) for k, v in backends.iteritems()\

                            if vm_fits_in_backend(v, vm))

    # Since we are conservatively updating backend resources on each

    # allocation, a backend may actually be able to host a vm (despite

    # the state of the backend in db)

    if not capable_backends:

        capable_backends = backends

    # Compute the scores for each backend

    backend_scores = [(back_id, backend_score(back_resources, vm))\

                      for back_id, back_resources in \

                      capable_backends.iteritems()]

    # Pick out the best

    result = min(backend_scores, key=lambda (b_id, b_score): b_score)

    backend_id = result[0]

    return backend_id

def vm_fits_in_backend(backend, vm):

    return backend['dfree'] > vm['disk'] and backend['mfree'] > vm['ram']

def backend_score(backend, flavor):

    mratio = 1 - (backend['mfree'] / backend['mtotal'])

    dratio = 1 - (backend['dfree'] / backend['dtotal'])

    cratio = (backend['pinst_cnt'] + 1) / (backend['ctotal'] * 4)

    return 0.7 * (mratio + dratio) * 0.3 * cratio