Failover to backup site in connection with triangular asynchronous replication

1. A method of handling failure of a primary group at a first data center that is part of plurality of data centers providing triangular asynchronous replication, the method comprising:

creating a data mirroring relationship between at least one storage volume at a second data center having a synchronous backup group that is part of the plurality of data centers and at least one storage volume at a third data center having an asynchronous backup group that is part of the plurality of data centers, wherein data writes to a local storage volume at the primary group begun after a first time and before a second time are associated with a first chunk of data and data writes to the local storage volume at the primary group begun after the second time are associated with a second chunk of data and wherein the at least one storage volume at the third data center stores data from the first chunk in response to receipt of a message from the primary group indicating that all data from the first chunk has been transmitted to the third data center;

maintaining failure recovery data that indicates data of the first and second chunks of data that has not yet been completely transferred to the third data center;

synchronizing the second and third data centers, wherein the failure recovery data indicates whether to synchronize the second and third data centers by providing initial data from the second data center to the third data center or by providing initial data from the third data center to the second data center and wherein synchronizing the second and third data centers includes discarding data corresponding to a particular chunk that has been received at the third data center if only some of the data for the particular chunk has been received at the third data center; and

after synchronizing the second and third data centers, resuming work at one of: the second data center and the third data center.

2. A method, according to claim 1 , further comprising:

waiting for consistency between the at least one storage volume at the second data center and the at least one storage volume at the third data center after resuming work.

3. A method, according to claim 1 , further comprising:

prior to creating a data mirroring relationship between at least one storage volume at the second data center and at least one storage volume at a third data center, making a local copy of data at the second data center and the third data center.

4. A method, according to claim 1 , further comprising:

performing a half swap operation on a data mirroring relationship between the at least one storage volume at the second data center and a corresponding at least one storage volume at the first data center, wherein the half swap operation causes the at least one storage volume at the second data center to reverse a direction of data mirroring irrespective of cooperation from the corresponding at least one storage volume at the first data center.

5. A method, according to claim 4 , wherein the at least one storage volume at the second data center accumulates data that may be transferred to the first data center if the first data center becomes operational.

6. A method, according to claim 1 , further comprising:

performing a half delete operation on a data mirroring relationship between the at least one storage volume at the third data center and a corresponding at least one storage volume at the first data center, wherein the half delete operation causes the at least one storage volume at the third data center to remove a data mirroring relationship with the at least one storage device at the first data center irrespective of cooperation from the corresponding at least one storage volume at the first data center.

7. A method, according to claim 1 , wherein, following receiving an acknowledgement from the third data center indicating that the first chunk of data has been received, subsequent data writes to the local storage volume at the primary group are associated with a third chunk of data, different from the first and second chunks of data.

8. A method, according to claim 1 , wherein data that is transmitted is in an inactive buffer and data being accumulated is in an active buffer and wherein, in response to receiving an acknowledgement from the at least one storage volume at the third data center that the first chunk of data has been received, the inactive buffer becomes the active buffer and the active buffer becomes the inactive buffer.

9. A method, according to claim 1 , further comprising:

performing a half swap operation on a data mirroring relationship between the at least one storage volume at the third data center and a corresponding at least one storage volume at the first data center, wherein the half swap operation causes the at least one storage volume at the third data center to reverse a direction of data mirroring irrespective of cooperation from the corresponding at least one storage volume at the first data center.

10. A method, according to claim 9 , wherein the at least one storage volume at the third data center accumulates data that may be transferred to the first data center if the first data center becomes operational.

11. A method, according to claim 1 , further comprising:

performing a half delete operation on a data mirroring relationship between the at least one storage volume at the second data center and a corresponding at least one storage volume at the first data center, wherein the half delete operation causes the at least one storage volume at the second data center to remove a data mirroring relationship with the at least one storage device at the first data center irrespective of cooperation from the corresponding at least one storage volume at the first data center.

12. Computer software, stored in a non-transitory computer readable medium, that handles failure of a primary group at a first data center that is part of plurality of data centers providing triangular asynchronous replication, comprising:

executable code that creates a data mirroring relationship between at least one storage volume at a second data center having a synchronous backup group that is part of the plurality of data centers and at least one storage volume at a third data center having an asynchronous backup group that is part of the plurality of data centers;

executable code that associates data writes to a local storage volume at the primary group begun after a first time and before a second time are associated with a first chunk of data and associates data writes to the local storage volume at the primary group begun after the second time to a second chunk of data;

executable code that causes the at least one storage volume at the third data center to store data from the first chunk in response to receipt of a message from the primary group indicating that all data from the first chunk has been transmitted to the third data center;

executable code that maintains failure recovery data that indicates data of the first and second chunks of data that has not yet been completely transferred to the third data center;

executable code that synchronizes the second and third data centers, wherein the failure recovery data indicates whether to synchronize the second and third data centers by providing initial data from the second data center to the third data center or by providing initial data from the third data center to the second data center and wherein executable code that synchronizes the second and third data centers discards data corresponding to a particular chunk that has been received at the third data center if only some of the data for the particular chunk has been received at the third data center; and

executable code that, after the second and third data centers are synchronized, causes work to be resumed at one of: the second data center and the third data center.

13. Computer software, according to claim 12 , further comprising:

executable code that waits for consistency between the at least one storage volume at the second data center and the at least one storage volume at the third data center after work is resumed.

14. Computer software, according to claim 12 , further comprising:

executable code that makes a local copy of data at the second data center and the third data center prior to creating a data mirroring relationship between at least one storage volume at the second data center and at least one storage volume at a third data center.

15. Computer software, according to claim 12 , further comprising:

executable code that performs a half swap operation on a data mirroring relationship between the at least one storage volume at the second data center and a corresponding at least one storage volume at the first data center, wherein the half swap operation causes the at least one storage volume at the second data center to reverse a direction of data mirroring irrespective of cooperation from the corresponding at least one storage volume at the first data center.

16. Computer software, according to claim 15 , wherein the at least one storage volume at the second data center accumulates data that may be transferred to the first data center if the first data center becomes operational.

17. Computer software, according to claim 12 , further comprising:

executable code that performs a half delete operation on a data mirroring relationship between the at least one storage volume at the third data center and a corresponding at least one storage volume at the first data center, wherein the half delete operation causes the at least one storage volume at the third data center to remove a data mirroring relationship with the at least one storage device at the first data center irrespective of cooperation from the corresponding at least one storage volume at the first data center.

18. Computer software, according to claim 12 , further comprising:

executable code that causes subsequent data writes to the local storage volume at the primary group to be associated with a third chunk of data, different from the first and second chunks of data, following receiving an acknowledgement from the third data center indicating that the first chunk of data has been received.

19. Computer software, according to claim 12 , wherein data that is transmitted is in an inactive buffer and data being accumulated is in an active buffer and wherein, in response to receiving an acknowledgement from the at least one storage volume at the third data center that the first chunk of data has been received, the inactive buffer becomes the active buffer and the active buffer becomes the inactive buffer.

20. Computer software, according to claim 12 , further comprising:

executable code that performs a half swap operation on a data mirroring relationship between the at least one storage volume at the third data center and a corresponding at least one storage volume at the first data center, wherein the half swap operation causes the at least one storage volume at the third data center to reverse a direction of data mirroring irrespective of cooperation from the corresponding at least one storage volume at the first data center.

21. Computer software, according to claim 20 , wherein the at least one storage volume at the third data center accumulates data that may be transferred to the first data center if the first data center becomes operational.

22. Computer software, according to claim 12 , further comprising:

executable code that performs a half delete operation on a data mirroring relationship between the at least one storage volume at the second data center and a corresponding at least one storage volume at the first data center, wherein the half delete operation causes the at least one storage volume at the second data center to remove a data mirroring relationship with the at least one storage device at the first data center irrespective of cooperation from the corresponding at least one storage volume at the first data center.