1. An operating method of a storage device including a nonvolatile memory and a memory controller configured to control the nonvolatile memory, the operating method comprising: selecting a submission queue from among a plurality of submission queues; fetching, from the selected submission queue, a command for accessing the storage device; executing the fetched command; outputting an execution result of the fetched command to a selected completion queue corresponding to the selected submission queue; and inhibiting, when a full completion queue exist, the selecting from a full submission queue corresponding to the full completion queue among the plurality of submission queues until the full completion queue becomes a normal completion queue, the plurality of submission queues being sequentially selected in accordance with a round robin scheduling, the plurality of submission queues having a plurality of weights, respectively, selection frequencies of the plurality of submission queues being based on the weights, the inhibiting including reducing a weight of the full submission queue based on determining the full completion queue exists, wherein the inhibiting includes adjusting a weight of the full submission queue to zero until the full submission queue becomes the normal completion queue, and wherein the inhibiting further includes increasing a weight of at least one normal submission queue among the plurality of submission queues, when the weight of the full submission queue is adjusted to zero.