A method and system for providing logically consistent backup copies of a logical unit provided by a data storage device, and for managing and maintaining a pool of increasingly stale backup copies within the data storage device. Backup logical units within the pool are continuously recycled, so that the least recently current backup logical unit is next synchronized and activated to receive mirror I/O requests. A trigger I/O request is provided to allow an application program running on a host computer to signal points within a stream of I/O requests that represent logically consistent states.