A virtual storage system comprises an agent to store a first table to map virtual storage segments to storage locations, and a controller to store a second table and to intermittently cause contents of the first table to be replaced by contents of the second table. During an I/O operation, one of the entries in the first table can be accessed by a host to determine one of the storage locations. Also, a method for performing an I/O operation on a virtual storage segment, defined by first table that maps a storage segment to a first storage location, comprises turning off I/O operations at the first storage location, identifying portions of the virtual storage segment to be effected during a write operation, storing a record of identified portions at a second table not the first table, and writing to portions of the second storage location associated with the identified portions.