Data replication among storage systems

• US 7,725,445 B2
• Filed: 11/17/2004
• Issued: 05/25/2010
• Est. Priority Date: 06/27/2003
• Status: Active Grant

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Takeda, Takahito, Sato, Takao, Arakawa, Hiroshi, Hirakawa, Yusuke
• Primary Examiner(s): PANNALA, SATHYANARAYA R
• Application Number:
  US10/992,432
• Publication Number:
  US 20050073887A1
• Time in Patent Office: 2,015 Days
• Field of Search: None
• US Class Current: 707/695
• CPC Class Codes: G06F 11/2064 while ensuring consistency
• CPC Class Codes: G06F 11/2071 using a plurality of controllers
• CPC Class Codes: G06F 11/2082 Data synchronisation
• CPC Class Codes: G06F 16/22 Indexing; Data structures therefor; Storage structures
• CPC Class Codes: G06F 2201/855 Details of asynchronous mirroring using a journal to transfer not-yet-mirrored changes
• CPC Class Codes: H04L 67/1095 Replication or mirroring of data, e.g. scheduling or transport for data synchronisation between network nodes
• CPC Class Codes: H04L 67/1097 for distributed storage of data in networks, e.g. transport arrangements for network file system [NFS], storage area networks [SAN] or network attached storage [NAS]
• CPC Class Codes: Y10S 707/99955 Archiving or backup

Abstract

A first storage system stores information relating to the updating of data stored in that system as a journal. More specifically, the journal is composed of a copy of data that was used for updating and update information such as a write command used during updating. Furthermore, the second storage system acquires the journal via a communication line between the first storage system and the second storage system. The second storage system holds a duplicate of the data held by the first storage system and updates the data corresponding to the data of the first storage system in the data update order of the first storage system by using the journal.

Claims

1. A storage system, outside of a host computer, coupled to another storage system, the storage system comprising:

a plurality of disk drives;

a plurality of first replication volumes, provided from areas of said plurality of drives, allocated to a first group, and storing a plurality of replicated data, corresponding to a plurality of data to be stored in a plurality of first volumes of the another storage system, wherein a consistency of the plurality of replicated data of the plurality of first replication volumes is maintained;

a first journal volume provided from areas of said plurality of drives and storing a plurality of journal data, received from the another storage system and related to the first group, the plurality of journal data stored in the first journal volume used to update a plurality of replicated data to be stored in at least one of the plurality of first replication volumes;

a plurality of second replication volumes, provided from areas of said plurality of drives, allocated to a second group, and storing replicated data, corresponding to data to be stored in a plurality of second volumes of the another storage system, wherein a consistency of the plurality of replicated data of the plurality of second replication volumes is maintained; and

a second journal volume provided from areas of said plurality of drives and storing a plurality of journal data, received from the another storage system and related to the second group, the plurality of journal data stored in the second journal volume used to update a plurality of replicated data to be stored in at least one of the plurality of second replication volumes,

wherein an oldest journal data stored in the first journal volume is read from the first journal volume,

wherein a replicated data corresponding to the oldest journal data stored in the first journal volume is written to one of the plurality of first replication volumes,

wherein an oldest journal data stored in the second journal volume is read from the second journal volume, and

wherein a replicated data corresponding to the oldest journal data stored in the second journal volume is written to one of the plurality of second replication volumes.

2. The storage system according to claim 1, wherein each of the plurality of journal data stored in the first journal volume includes an information of a write order in the first group and a replicated data.

3. The storage system according to claim 1, wherein each of the plurality of journal data stored in the first journal volume includes an update number in the first group.

4. The storage system according to claim 1, wherein each of the plurality of journal data stored in the first journal volume includes an information of a write order in the first group, and

wherein each of the plurality of journal data stored in the second journal volume includes an information of a write order in the second group.

5. The storage system according to claim 1, wherein each of the plurality of journal data stored in the first journal volume includes an update number in the first group and a replicated data, and

wherein each of the plurality of journal data stored in the second journal volume includes an update number in the second group and a replicated data.

6. The storage system according to claim 1, further comprising:

a controller coupled to the first journal volume and the second journal volume,

wherein the controller controls to read the oldest journal data stored in the first journal volume from the first journal volume,

wherein the controller controls to store the replicated data, corresponding to the oldest journal data stored in the first journal volume, in one of the plurality of first replication volumes,

wherein the controller controls to free a part of the first journal volume after storing the replicated data, corresponding to the oldest journal data stored in the first journal volume, in the one of the plurality of first replication volumes,

wherein the controller controls to read the oldest journal data from the second journal volume,

wherein the controller controls to store the replicated data, corresponding to the oldest journal data stored in the second journal volume, in one of the plurality of second replication volumes, and

wherein the controller controls to free a part of the second journal volume after storing the replicated data, corresponding to the oldest journal data stored in the second journal volume, in the one of the plurality of second replication volumes.

7. The storage system according to claim 1,

wherein a part of the first journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the first journal volume, to the one of the plurality of first replication volumes, and

wherein a part of the second journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the second journal volume, to the one of the plurality of second replication volumes.

8. A storage system coupled to another storage system coupled to a host computer, the storage system comprising:

a plurality of disk drives;

a plurality of first replication volumes, provided from areas of said plurality of drives, assigned to a first group in which a data consistency is maintained across the plurality of first replication volumes, for storing a plurality of replicated data, corresponding to a plurality of data stored in a plurality of first volumes of the another storage system;

a first journal volume provided from areas of said plurality of drives and storing a plurality of first journal data, received from the another storage system and related to the first group, the plurality of first journal data used to update a plurality of replicated data to be stored in at least one of the plurality of first replication volumes;

a plurality of second replication volumes, provided from areas of said plurality of drives, assigned to a second group in which a data consistency is maintained across the plurality of second replication volumes, for storing replicated data, corresponding to data stored in a plurality of second volumes of the another storage system; and

a second journal volume provided from areas of said plurality of drives and storing a plurality of second journal data, received from the another storage system and related to the second group, the plurality of second journal data used to update a plurality of replicated data to be stored in at least one of the plurality of second replication volumes,

wherein an oldest journal data stored in the first journal volume is read from the first journal volume,

wherein a replicated data corresponding to the oldest journal data stored in the first journal volume is written to one of the plurality of first replication volumes,

wherein an oldest journal data stored in the second journal volume is read from the second journal volume, and

wherein a replicated data corresponding to the oldest journal data stored in the second journal volume is written to one of the plurality of second replication volumes.

9. The storage system according to claim 8, wherein each of the plurality of first journal data includes an information of a write order in the first group and a replicated data.

10. The storage system according to claim 8, wherein each of the plurality of first journal data includes an update number in the first group.

11. The storage system according to claim 8, wherein each of the plurality of first journal data includes an information of a write order in the first group, and

wherein each of the plurality of second journal data includes an information of a write order in the second group.

12. The storage system according to claim 8, wherein each of the plurality of first journal data includes an update number in the first group and a replicated data, and

wherein each of the plurality of second journal data includes an update number in the second group and a replicated data.

13. The storage system according to claim 8, further comprising:

a controller coupled to the first journal volume and the second journal volume,

wherein the controller controls to read the oldest journal data stored in the first journal volume from the first journal volume,

wherein the controller controls to store the replicated data, corresponding to the oldest journal data stored in the first journal volume, in one of the plurality of first replication volumes,

wherein the controller controls to free a part of the first journal volume after storing the replicated data, corresponding to the oldest journal data stored in the first journal volume, in the one of the plurality of first replication volumes,

wherein the controller controls to read the oldest journal data stored in the second journal volume from the second journal volume,

wherein the controller controls to store the replicated data, corresponding to the oldest journal data stored in the second journal volume, in one of the plurality of second replication volumes, and

wherein the controller controls to free a part of the second journal volume after storing the replicated data, corresponding to the oldest journal data stored in the second journal volume, in the one of the plurality of second replication volumes.

14. The storage system according to claim 8,

wherein a part of the first journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the first journal volume, to the one of the plurality of first replication volumes, and

wherein a part of the second journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the second journal volume, to the one of the plurality of second replication volumes.

15. A storage system coupled to another storage system coupled to a host computer, the storage system comprising:

a plurality of disk drives;

a first journal volume provided from areas of said plurality of drives and storing a plurality of journal data received from the another storage system;

a plurality of first replication volumes provided from areas of said plurality of drives and storing a plurality of replicated data, corresponding to the plurality of journal data stored in the first journal volume and corresponding to a plurality of data stored in a plurality of first volumes of the another storage system, and the plurality of first replication volumes allocated to a first group in which a consistency of the plurality of replicated data of the plurality of first replication volumes is maintained;

a second journal volume provided from areas of said plurality of drives and storing a plurality of journal data received from the another storage system; and

a plurality of second replication volumes provided from areas of said plurality of drives and storing a plurality of replicated data, corresponding to the plurality of journal data stored in the second journal volume and corresponding to a plurality of data stored in a plurality of second volumes of the another storage system, and the plurality of second replication volumes allocated to a second group in which a consistency of the plurality of replicated data of the plurality of second replication volumes is maintained,

wherein an oldest journal data stored in the first journal volume is read from the first journal volume,

wherein a replicated data corresponding to the oldest journal data stored in the first journal volume is written to one of the plurality of first replication volumes,

wherein an oldest journal data stored in the second journal volume is read from the second journal volume, and

wherein a replicated data corresponding to the oldest journal data stored in the second journal volume is written to one of the plurality of second replication volumes.

16. The storage system according to claim 15, wherein each of the plurality of journal data stored in the first journal volume includes an information of a write order in the first group and a replicated data.

17. The storage system according to claim 15, wherein each of the plurality of journal data stored in the first journal volume includes an update number in the first group.

18. The storage system according to claim 15, wherein each of the plurality of journal data stored in the first journal volume includes an information of a write order in the first group, and

wherein each of the plurality of journal data stored in the second journal volume includes an information of a write order in the second group.

19. The storage system according to claim 15, wherein each of the plurality of journal data stored in the first journal volume includes an update number in the first group and a replicated data, and

wherein each of the plurality of journal data stored in the second journal volume includes an update number in the second group and a replicated data.

20. The storage system according to claim 15, further comprising:

a controller coupled to the first journal volume and the second journal volume,

wherein the controller controls to read the oldest journal data stored in the first journal volume from the first journal volume,

wherein the controller controls to store the replicated data, corresponding to the oldest journal data stored in the first journal volume, in one of the plurality of first replication volumes,

wherein the controller controls to free a part of the first journal volume after storing the replicated data, corresponding to the oldest journal data stored in the first journal volume, in the one of the plurality of first replication volumes,

wherein the controller controls to read the oldest journal data stored in the second journal volume from the second journal volume,

wherein the controller controls to store the replicated data, corresponding to the oldest journal data stored in the second journal volume, in one of the plurality of second replication volumes, and

wherein the controller controls to free a part of the second journal volume after storing the replicated data, corresponding to the oldest journal data stored in the at least one second journal volume, in the one of the plurality of second replication volumes.

21. The storage system according to claim 15,

wherein a part of the first journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the first journal volume, to the one of the plurality of first replication volumes, and

wherein a part of the second journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the second journal volume, to the one of the plurality of second replication volumes.

22. Controller used in a storage system, the controller comprising:

a memory;

at least one first port coupled to another storage system coupled to a host computer; and

at least one second port coupled to a plurality of first replication volumes, a first journal volume, a plurality of second replication volumes and a second journal volume, each of which is provided from areas of a plurality of disk drives,

wherein the plurality of first replication volumes are allocated to a first group and store a plurality of replicated data, corresponding to a plurality of data to be stored in a plurality of first volumes of the another storage system,

wherein a consistency of the plurality of replicated data of the plurality of first replication volumes is maintained,

wherein the first journal volume store a plurality of journal data, received from the another storage system and related to the first group, the plurality of journal data stored in the first journal volume used to update a plurality of replicated data to be stored in at least one of the plurality of first replication volumes,

wherein the plurality of second replication volumes are allocated to a second group and store replicated data, corresponding to data to be stored in a plurality of second volumes of the another storage system, wherein a consistency of the plurality of replicated data of the plurality of second replication volumes is maintained,

wherein the second journal volume store a plurality of journal data, received from the another storage system and related to the second group, the plurality of journal data stored in the second journal volume used to update a plurality of replicated data to be stored in at least one of the plurality of second replication volumes,

wherein an oldest journal data stored in the first journal volume is read from the first journal volume,

wherein a replicated data corresponding to the oldest journal data stored in the first journal volume is written to one of the plurality of first replication volumes,

wherein an oldest journal data stored in the second journal volume is read from the second journal volume, and

wherein a replicated data corresponding to the oldest journal data stored in the second journal volume is written to one of the plurality of second replication volumes.

23. The controller according to claim 22, wherein each of the plurality of journal data stored in the first journal volume includes an information of a write order in the first group and a replicated data.

24. The controller according to claim 22, wherein each of the plurality of journal data stored in the first journal volume includes an update number in the first group.

25. The controller according to claim 22, wherein each of the plurality of journal data stored in the first journal volume includes an information of a write order in the first group, and

wherein each of the plurality of journal data stored in the second journal volume includes an information of a write order in the second group.

26. The controller according to claim 22, wherein each of the plurality of journal data stored in the first journal volume includes an update number in the first group and a replicated data, and

wherein each of the plurality of journal data stored in the second journal volume includes an update number in the second group and a replicated data.

27. The controller according to claim 22, wherein the controller controls to read the oldest journal data stored in the first journal volume from the first journal volume,

wherein the controller controls to store the replicated data, corresponding to the oldest journal data stored in the first journal volume, in one of the plurality of first replication volumes,

wherein the controller controls to free a part of the first journal volume after storing the replicated data, corresponding to the oldest journal data stored in the first journal volume, in the one of the plurality of first replication volumes,

wherein the controller controls to read the oldest journal data stored in the second journal volume from the second journal volume,

wherein the controller controls to store the replicated data, corresponding to the oldest journal data stored in the second journal volume, in one of the plurality of second replication volumes, and

wherein the controller controls to free a part of the second journal volume after storing the replicated data, corresponding to the oldest journal data stored in the second journal volume, in the one of the plurality of second replication volumes.

28. The controller according to claim 22,

wherein a part of the first journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the first journal volume, to the one of the plurality of first replication volumes, and

wherein a part of the second journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the second journal volume, to the one of the plurality of second replication volumes.

29. A controller used in a storage system, the controller comprising:

a memory;

at least one first port coupled to another storage system coupled to a host computer; and

at least one second port coupled to a plurality of first replication volumes, a first journal volume, a plurality of second replication volumes and a second journal volume, each of which is provided from areas of a plurality of disk drives,

wherein the plurality of first replication volumes are assigned to a first group in which a data consistency is maintained across the plurality of first replication volumes, and are used for storing a plurality of replicated data, corresponding to a plurality of data stored in a plurality of first volumes of the another storage system,

wherein the first journal volume is used for storing a plurality of first journal data, received from the another storage system and related to the first group, the plurality of first journal data used to update a plurality of replicated data to be stored in at least one of the plurality of first replication volumes,

wherein the plurality of second replication volumes are assigned to a second group in which a data consistency is maintained across the plurality of second replication volumes, and are used for storing replicated data, corresponding to data stored in a plurality of second volumes of the another storage system,

wherein the second journal volume is used for storing a plurality of second journal data, received from the another storage system and related to the second group, the plurality of second journal data used to update a plurality of replicated data to be stored in at least one of the plurality of second replication volumes,

wherein an oldest journal data stored in the first journal volume is read from the first journal volume,

wherein a replicated data corresponding to the oldest journal data stored in the first journal volume is written to one of the plurality of first replication volumes,

wherein an oldest journal data stored in the second journal volume is read from the second journal volume, and

wherein a replicated data corresponding to the oldest journal data stored in the second journal volume is written to one of the plurality of second replication volumes.

30. The controller according to claim 29, wherein each of the plurality of first journal data includes an information of a write order in the first group and a replicated data.

31. The controller according to claim 29, wherein each of the plurality of first journal data includes an update number in the first group.

32. The controller according to claim 29, wherein each of the plurality of first journal data includes an information of a write order in the first group, and

wherein each of the plurality of second journal data includes an information of a write order in the second group.

33. The controller according to claim 29, wherein each of the plurality of first journal data includes an update number in the first group and a replicated data, and

wherein each of the plurality of second journal data includes an update number in the second group and a replicated data.

34. The controller according to claim 29, wherein the controller controls to read the oldest journal data stored in the first journal volume from the first journal volume,

wherein the controller controls to store the replicated data, corresponding to the oldest journal data stored in the first journal volume, in one of the plurality of first replication volumes,

wherein the controller controls to free a part of the first journal volume after storing the replicated data, corresponding to the oldest journal data stored in the first journal volume, in the one of the plurality of first replication volumes,

wherein the controller controls to read the oldest journal data stored in the second journal volume from the second journal volume,

wherein the controller controls to store the replicated data, corresponding to the oldest journal data stored in the second journal volume, in one of the plurality of second replication volumes, and

wherein the controller controls to free a part of the second journal volume after storing the replicated data, corresponding to the oldest journal data stored in the second journal volume, in the one of the plurality of second replication volumes.

35. The controller according to claim 29,

wherein a part of the first journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the first journal volume, to the one of the plurality of first replication volumes, and

wherein a part of the second journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the second journal volume, to the one of the plurality of second replication volumes.

36. A controller used in a storage system and coupled to another storage system coupled to a host computer, the controller comprising:

a computer program stored on a computer readable storage medium and implemented in the controller, said computer program comprising;

code controlling to transfer at least one journal data, received from the another storage system, to a first journal volume provided from areas of a plurality of disk drives;

code controlling to read the at least one journal data, transferred to the first journal volume, from the first journal volume;

code controlling to transfer at least one replicated data, corresponding to the at least one journal data read from the first journal volume and corresponding to at least one data stored in at least one of a plurality of first volumes of the another storage system, to at least one of a plurality of first replication volumes, the plurality of first replication volumes provided from areas of said plurality of drives and allocated to a first group in which a consistency of a plurality of replicated data of the plurality of first replication volumes is maintained;

code controlling to transfer at least one journal data, received from the another storage system, to a second journal volume provided from areas of said plurality of drives;

code controlling to read the at least one journal data, transferred to the second journal volume, from the second journal volume; and

code controlling to transfer at least one replicated data, corresponding to the at least one journal data read from the second journal volume and corresponding to at least one data stored in at least one of a plurality of second volumes of the another storage system, to at least one of a plurality of second replication volumes, the plurality of second replication volumes provided from areas of said plurality of drives and allocated

to a second group in which a consistency of a plurality of replicated data of the plurality of second replication volumes is maintained,

wherein an oldest journal data stored in the first journal volume is read from the first journal volume,

wherein a replicated data corresponding to the oldest journal data stored in the first journal volume is written to one of the plurality of first replication volumes,

wherein an oldest journal data stored in the second journal volume is read from the second journal volume, and

wherein a replicated data corresponding to the oldest journal data stored in the second journal volume is written to one of the plurality of second replication volumes.

37. The controller according to claim 36, wherein each of the plurality of journal data stored in the first journal volume includes an information of a write order in the first group and a replicated data.

38. The controller according to claim 36, wherein each of the plurality of journal data stored in the first journal volume includes an update number in the first group.

39. The controller according to claim 36, wherein each of the plurality of journal data stored in the first journal volume includes an information of a write order in the first group, and

wherein each of the plurality of journal data stored in the second journal volume includes an information of a write order in the second group.

40. The controller according to claim 36, wherein each of the plurality of journal data stored in the first journal volume includes an update number in the first group and a replicated data, and

wherein each of the plurality of journal data stored in the second journal volume includes an update number in the second group and a replicated data.

41. The controller according to claim 36, wherein the computer program further comprises:

code controlling to read the oldest journal data, stored in the first journal volume, from the first journal volume,

code controlling to transfer the replicated data, corresponding to the oldest journal data stored in the first journal volume, to one of the plurality of first replication volumes,

code controlling to free a part of the first journal volume after transferring the replicated data, corresponding to the oldest journal data stored in the first journal volume, to the one of the plurality of first replication volumes,

code controlling to read the oldest journal data, stored in the second journal volume, from the second journal volume,

code controlling to transfer the replicated data, corresponding to the oldest journal data stored in the second journal volume, to one of the plurality of second replication volumes, and

code controlling to free a part of the second journal volume after transferring the replicated data, corresponding to the oldest journal data stored in the second journal volume, to the one of the plurality of second replication volumes.

42. The controller according to claim 36,

wherein a part of the first journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the first journal volume, to the one of the plurality of first replication volumes, and

wherein a part of the second journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the second journal volume, to the one of the plurality of second replication volumes.

43. A computer program stored on a computer readable storage medium and implemented in a controller, the controller used in a storage system and coupled to another storage system coupled to a host computer, the computer program comprising:

code controlling to transfer at least one journal data, received from the another storage system, to a first journal volume provided from areas of a plurality of disk drives;

code controlling to read the at least one journal data, transferred to the first journal volume, from the first journal volume;

code controlling to transfer at least one replicated data, corresponding to the at least one journal data read from the first journal volume and corresponding to at least one data stored in at least one of a plurality of first volumes of the another storage system, to at least one of a plurality of first replication volumes, the plurality of first replication volumes provided from areas of said plurality of drives and allocated to a first group in which a consistency of a plurality of replicated data of the plurality of first replication volumes is maintained;

code controlling to transfer at least one journal data, received from the another storage system, to a second journal volume provided from areas of said plurality of drives;

code controlling to read the at least one journal data, transferred to the second journal volume, from the second journal volume; and

code controlling to transfer at least one replicated data, corresponding to the at least one journal data read from the second journal volume and corresponding to at least one data stored in at least one of a plurality of second volumes of the another storage system, to at least one of a plurality of second replication volumes, the plurality of second replication volumes provided from areas of said plurality of drives and allocated to a second group in which a consistency of a plurality of replicated data of the plurality of second replication volumes is maintained,

wherein an oldest journal data stored in the first journal volume is read from the first journal volume,

wherein a replicated data corresponding to the oldest journal data stored in the first journal volume is written to one of the plurality of first replication volumes,

wherein an oldest journal data stored in the second journal volume is read from the second journal volume, and

wherein a replicated data corresponding to the oldest journal data stored in the second journal volume is written to one of the plurality of second replication volumes.

44. The computer program according to claim 43, wherein each of the plurality of journal data stored in the first journal volume includes an information of a write order in the first group and a replicated data.

45. The computer program according to claim 43, wherein each of the plurality of journal data stored in the first journal volume includes an update number in the first group.

46. The computer program according to claim 43, wherein each of the plurality of journal data stored in the first journal volume includes an information of a write order in the first group, and

wherein each of the plurality of journal data stored in the second journal volume includes an information of a write order in the second group.

47. The computer program according to claim 43, wherein each of the plurality of journal data stored in the first journal volume includes an update number in the first group and a replicated data, and

wherein each of the plurality of journal data stored in the second journal volume includes an update number in the second group and a replicated data.

48. The computer program according to claim 43, wherein the computer program further comprises:

code controlling to read the oldest journal data, stored in the first journal volume, from the first journal volume;

code controlling to transfer the replicated data, corresponding to the oldest journal data stored in the first journal volume, to one of the plurality of first replication volumes;

code controlling to free a part of the first journal volume after transferring the replicated data, corresponding to the oldest journal data stored in the first journal volume, to the one of the plurality of first replication volumes;

code controlling to read the oldest journal data, stored in the second journal volume, from the second journal volume;

code controlling to transfer the replicated data, corresponding to the oldest journal data stored in the second journal volume, to one of the plurality of second replication volumes; and

code controlling to free a part of the second journal volume after transferring the replicated data, corresponding to the oldest journal data stored in the second journal volume, to the one of the plurality of second replication volumes.

49. The computer program according to claim 43,

wherein a part of the first journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the first journal volume, to the one of the plurality of first replication volumes, and

wherein a part of the second journal volume is freed after writing the replicated data, corresponding to the oldest journal data stored in the second journal volume, to the one of the plurality of second replication volumes.