Patent ID: 7512835

Claim:
A method of recovery from a data storage system failure in a data storage system having a host computer writing data to a first storage unit associated with a first storage controller synchronously mirroring the data to a second storage unit associated with a second storage controller asynchronously mirroring the data to a third storage unit, the second storage controller comprising a first map identifying tracks storing the mirrored data which has been received on the second storage unit by the synchronous mirroring of the data from the first storage unit and a second map identifying the tracks storing changed data from a point in time where the host computer begins writing the data updates directly to the second storage unit, the method comprising: detecting a failure associated with the first storage unit; writing data updates directly from the host to the second storage unit; correcting the failure associated with the first storage unit; suspending the asynchronous mirroring of the data updates from the second storage unit to the third storage unit before the data updates are synchronously mirrored from the second storage unit to the first storage unit; re-establishing the synchronous mirroring of the data from the first storage unit to the second storage unit; resuming the writing of the data from the host computer to the first storage unit; and re-establishing the asynchronous mirroring of data from the second storage unit to the third storage unit by: comparing the second map to the first map and adding the identity of the tracks containing the changed data, as identified by the second map but not identified by the first map, to the first map; and asynchronously mirroring the data updates on the tracks then identified by the first map from the second storage unit to the third storage unit; and synchronously mirroring the data updates from the second storage unit to the first storage unit.