US Patent 7571293 Emulation of point-in-time data copying operations

A host computer system (host) includes an application that generates I/O commands for a first type of point-in-time (PIT) copy operation, and an I/O subsystem coupled to a data storage system capable of performing a second type of PIT copy operation. The host processes the I/O commands as part of an emulation of the first type of PIT copy operation, including determining whether the I/O commands require modification for the emulation, and if so whether they are of a back-end type or an out-of-band type. The I/O commands that do not require modification are processed without modification via the I/O subsystem. For the back-end I/O commands, (i) an intercept condition is set, (ii) the I/O commands are processed via the I/O subsystem, and (iii) upon completion with the intercept condition set, the results are modified so as to indicate to the application that the first type of PIT copy operation can be performed on the storage volumes. For the out-of-band I/O commands, (i) corresponding commands of the second type of PIT copy operation are processed via the I/O subsystem in lieu of the out-of-band I/O commands, and (ii) upon completion of the substituted I/O commands, corresponding results are returned to the application as though the out-of-band I/O commands had been processed by the I/O subsystem. Using this technique, a host that initiates only PIT copying operations of the first type can be used with storage systems that support second-type PIT copying operations.