System and method for the opportunistic allocation of copies of data across data storage resources in file systems. Embodiments may opportunistically and transparently trade storage space in a file system for performance. In embodiments, when data is stored to the file system, copies of the data may be opportunistically stored to two or more data storage resources. The data may then be opportunistically accessed from a less active resource if the resource storing a primary copy of the data is busy. The opportunistically stored copies of data remain online in the file system. In hierarchical file systems or tiered storage hierarchies, embodiments may allow higher-level storage tiers to “borrow” performance from lower-level tiers by opportunistically allocating copies of data from a higher-level tier within a lower-level tier. Copies of data stored on a data storage resource may be deleted if storage space is needed on that data storage resource.