Method, system, and program for accessing data from storage systems

• US 7,039,657 B1
• Filed: 11/09/1999
• Issued: 05/02/2006
• Est. Priority Date: 11/09/1999
• Status: Expired due to Fees

• Current Assignee: International Business Machines Corporation
• Original Assignee: International Business Machines Corporation
• Inventors: Kishi, Gregory Tad, Bish, Thomas William
• Primary Examiner(s): Ali, Mohammad
• Application Number:
  US09/436,506
• Time in Patent Office: 2,366 Days
• Field of Search: 707 1- 10, 707100-1041, 707200-205, 711/166, 711/163.15, 711111-113, 711133-151, 711/163, 710/74, 709/227, 709104-105
• US Class Current: 1/1
• CPC Class Codes: G06F 11/2056 by mirroring
• CPC Class Codes: G06F 12/0866 for peripheral storage systems, e.g. disk cache
• CPC Class Codes: G06F 2201/815 Virtual middleware or OS functionality using virtual machines to implement generic software techniques for error detection or fault masking G06F11/1484
• CPC Class Codes: Y10S 707/99954 Version management

Abstract

Disclosed is a system, method, and program for accessing a data set, e.g., volume, file, from one of two storage devices. Each storage device includes a copy of the data set. A flag is maintained for each storage device indicating whether a previous access attempt of the data set from the storage device failed. The storage device having the flag indicating that no previous access attempt failed is selected if the flag for the other storage device indicates that one previous access attempt of the data set from the storage device failed. The data set is accessed from the selected storage device.

Claims

1. A method for accessing a plurality of data sets from one of a first, second, third and fourth storage devices, including a copy of the data set, comprising:

maintaining a flag for each of the first and second storage devices indicating whether a previous access attempt of the data set from the first and second storage devices failed;

maintaining a data level for each of the plurality of data sets in each of the first and second storage devices indicating a number of times the data set has been updated, wherein there is a separate data level for each copy of one data set in the first and second storage devices and wherein the data levels for the first and second storage devices having copies of one data set are capable of being equal;

receiving a request to one data set;

accessing the data set from one of the third and fourth storage devices if the data set is in one of the third and fourth storage devices;

selecting the first or second storage device having a higher data level for the requested data set if the data levels are not equal and if the data set is not in one of the third and fourth storage devices;

selecting the first or second storage device having the flag indicating that no previous access attempt failed if the flag for the other of the first or second storage device indicates that one previous access attempt of the data set from the first or second storage device failed and if the data levels are equal and if the data set is not in one of the third and fourth storage devices;

accessing the data set from the selected first or second storage device;

copying the data set from the first storage device to the third storage device when accessing the data set from the first storage device; and

copying the data set from the second storage device to the fourth storage device when accessing the data set from the first storage device.

2. The method of claim 1, further comprising using a selection criteria to access one of the first and second storage devices that is unrelated to a value of the flag if the flags for both storage devices have the same value.

3. The method of claim 1, further comprising synchronizing the data set on both the first and second storage devices after accessing the data set from the selected storage device.

4. The method of claim 1, where the flag and data level are maintained for each data set in the first and second storage devices and wherein the first and second storage devices have the same data sets.

5. The method of claim 1, wherein the step of accessing the data set comprises recalling the data set and whey the flag indicates whether a recall attempt of the data failed, further comprising:

scheduling a write operation to copy the data set from the third storage device to the first storage device;

receiving a request to access the data set after scheduling the write operation; and

recalling the data set from the third storage device if the scheduled write operation has not yet copied the requested data set to the first storage device, wherein the steps of selecting one of the first and second storage devices to access the data set and accessing the data set occurs if the scheduled write operation of the data set to the first storage device completed.

6. The method of claim 5, further comprising randomly selecting one of the first and second storage devices from which to recall the data set if the requested data set has been copied to the first and second storage devices as a result of the scheduled write operation and if the flags for both storage devices have the same value.

7. The method of claim 1, wherein the data level is maintained for the data set in both the third and fourth storage devices indicating a version of the data set, further comprising determining whether the data set is in both the third and fourth storage devices, wherein accessing the data set from one of the third and fourth storage devices comprises accessing the data set from one of the third and fourth storage devices having the highest data level for the data set.

8. The method of claim 7, further comprising randomly selecting one of the third and fourth storage devices from which to access the data set if the data levels of the data set at both the third and fourth storage devices have the same value.

9. A system for accessing a data set from one of a first, second, third, and fourth storage devices, including a copy of the data set, comprising:

means for maintaining a flag for each of the first and second storage devices indicating whether a previous access attempt of the data set from the first and second storage devices failed;

means for maintaining a data level for each of the plurality of data sets in each of the first and second storage devices indicating a number of times the data set has been updated, wherein there is a separate data level for each copy of one data set in the first and second storage devices and wherein the data levels for the first and second storage devices having copies of one data set are capable of being equal;

means for receiving a request to one data set;

means for accessing the data set from one of the third and fourth storage devices if the data set is in one of the third and fourth storage devices;

means for selecting the first or second storage device having a higher data level if the data levels are not equal and if the data set is not in one of the third and fourth storage devices;

means for selecting the first or second storage device having the flag indicating that no previous access attempt failed if the flag for the other of the first or second storage device indicates that one previous access attempt of the data set from the first or second storage device failed and if the data levels are equal and if the data set is not in one of the third and fourth storage devices;

means for accessing the data set from the selected first or second storage devices;

means for copying the data set from the first storage device to the third storage device when accessing the data set from the first storage device; and

means for copying the data set from the second storage device to the fourth storage device when accessing the data set from the first storage device.

10. The system of claim 9, fibs comprising means for using a selection criteria to access one of the first and second storage devices that is unrelated to a value of the flag if the flags for both storage devices have the same value.

11. The system of claim 9, further comprising means for synchronizing the data set on both the first and second storage devices after accessing the data set from the selected storage device.

12. The system of claim 9, wherein the flag and data levels are maintained for each data set in the first and second storage devices and wherein the first and second storage devices have the same data sets.

13. The system of claim 9, wherein the means for accessing the data set comprises recalling the data set and wherein the flag indicates whether a recall attempt of the data failed, further comprising:

means for scheduling a write operation to copy the data set from the third storage device to the first storage device;

means for receiving a request to access the data set after scheduling the write operation; and

means for recalling the data set from the third storage device if the scheduled write operation has not yet copied the requested data set to the first storage device, wherein the steps of selecting one of the first and second storage devices to access the data set and accessing the data set occurs if the scheduled write operation of the data set to the first storage device completed.

14. The system of claim 13, further comprising for randomly selecting one of the first and second storage devices from which to recall the data set if the requested data set has been copied to the first and second storage devices as a result of the scheduled write operation and if the flags for both storage devices have the same value.

15. The system of claim 9, further comprising mums for determining whether the data set is in both the third and fourth storage devices, wherein the mean for accessing the data set from one of the third and fourth storage devices comprises accessing the data set from one of the third and fourth storage devices having the highest data level for the data set.

16. The system of claim 15, further comprising means for randomly selecting one of the third and fourth storage devices from which to access the data set if the data levels of the data set at both the third and fourth storage devices have the same value.

17. An article of manufacture for use in programming at least one processing unit to access a data set from one of a first, second, third, and fourth storage devices, including a copy of the data set, the article of manufacture comprising computer readable storage media including at least one computer program embedded therein that causes the at least one processing unit to perform:

maintaining a flag for each of the first and second storage devices indicating whether a previous access attempt of the data set from the first and second storage devices failed;

maintaining a data level for each of the plurality of data sets in each of the first and second storage devices indicating a number of times the data set has been updated, wherein there is a separate data level for each copy of one data set in different the first and second storage devices and wherein the data levels for the first and second storage devices having copies of one data set are capable of being equal;

receiving a request to one data set;

accessing the data set from one of the third and fourth storage devices if the data set is in one of the third and fourth storage devices;

selecting the first or second storage device having a higher data level if the data levels are not equal and if the data set is not in one of the third and fourth storage devices;

selecting the first or second storage device having the flag indicating that no previous access attempt failed if the flag for the other of the first or second storage device indicates that one previous access attempt of the data set from the first or second storage device failed and if the data levels are equal and if the data set is not in one of the third and fourth storage devices; and

accessing the data set from the selected first or second storage device;

copying the data set from the first storage device to the third storage device when accessing the data set from the first storage device; and

copying the data set from the second storage device to the fourth storage device when accessing the data set from the first storage device.

18. The article of manufacture of claim 17, further comprising using a selection criteria to access one of the that and second storage devices that is unrelated to a value of the flag if the flags for both storage devices have the same value.

19. The article of manufacture of claim 17, further comprising synchronizing the data set on both the first and second storage devices after accessing the data set from the selected storage device.

20. The article of manufacture of claim 17, wherein the flag and data levels are maintained for each data set in the first and second storage devices and wherein the first and second storage devices have the same data sets.

21. The article of manufacture of claim 17, wherein the step of accessing the data set comprises recalling the data set and wherein the flag indicates whether a recall attempt of the data failed, further comprising:

scheduling a write operation to copy the data set from the third storage device to the first storage device;

receiving a request to access the data set after scheduling the write operation; and

recalling the data set from the third storage device if the scheduled write operation has not yet copied the required data set to the first storage device, wherein the steps of selecting one of the first and second storage devices to access the data set and accessing the data set occurs if the scheduled write operation of the data set to the first storage device completed.

22. The article of manufacture of claim 21, further comprising randomly selecting one of the first and second storage devices from which to recall the data set if the requested data set has been copied to the first and second storage devices as a result of the scheduled write operation and if the flap for both storage devices have the same value.

23. The article of manufacture of claim 17, comprises accessing the data set from one of the third and fourth storage devices having the highest data level for the data set.

24. The article of manufacture of claim 17, further comprising randomly selecting one of the third fourth storage devices from which to access the data set if the data levels of the data set at both the third and fourth storage devices have the same value.