US Patent 7779296 Storing data replicas remotely

Storing data is disclosed. All base data is copied from a first storage device to a master set on a second storage device. A request is received to write data to a specified location on the first storage device. An indication is stored in a list that the data at the specified location has changed. At a specified base time and using the stored indication, a replica of the data stored at the specified location as of that time is sent to a remote host associated with the second storage device. The remote host is configured to make and store a copy of the base data stored in the specified location in the master set prior to merging the replica data into the master set.