1. A method for inventory management, comprising: deploying a first mobile drive unit having first dimensions in a workspace; deploying a second mobile drive unit having second dimensions in the workspace, wherein the first and second dimensions are different; deploying a management module configured to maintain a three-dimensional reservation space in which the first mobile drive unit is operable to reserve a first three-dimensional reservation volume corresponding to the first dimensions and the second mobile drive unit is operable to reserve a second three-dimensional reservation volume corresponding to the second dimensions; and wherein the first and second mobile drive units are operable to transport inventory items to a plurality of inventory stations in the same workspace.