Read ahead storage control

• US 20080005481A1
• Filed: 06/30/2006
• Published: 01/03/2008
• Est. Priority Date: 06/30/2006
• Status: Active Grant

• Current Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Original Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Inventors: Walker, Michael D.
• Granted Patent: US 7,996,623 B2
• Time in Patent Office: Days
• US Class Current: 711/137
• CPC Class Codes: G06F 12/0862 with prefetch
• CPC Class Codes: G06F 12/0866 for peripheral storage systems, e.g. disk cache

Abstract

Method and apparatus for managing the storage of data in a cache memory by placing pending read requests for sequential data in a dedicated read ahead stream control (RASC) data structure, and further configured for dynamically switching both ways, in response to data stored in the RASC, between speculative non-requested read ahead data streaming to read behind stream locking on the read requests in the RASC.

Claims

1. An apparatus comprising a cache manager configured for placing pending read requests for sequential data in a read ahead stream control (RASC) data structure, and further configured for storing sequential data in a cache by switching, in response to data stored in the RASC, from speculative non-requested read ahead data to read behind stream locking on the read requests in the RASC.

2. The apparatus of claim 1 wherein the data stored in the RASC indicates that a particular read request has overrun a contemporaneous non-requested read ahead data.

3. The apparatus of claim 2 wherein the RASC compares a logical block address (LBA) associated with the read request to an LBA associated with the contemporaneous non-requested read ahead data.

4. The apparatus of claim 3 wherein the cache manager switches from caching non-requested read ahead data to read behind stream locking of requested data when the read request LBA is greater than the non-requested read ahead data LBA.

5. The apparatus of claim 4 wherein the cache manager transfers non-requested read ahead data to the cache memory in relation to a time parameter and a locality parameter associated with a data structure of which the requested data forms a part.

6. The apparatus of claim 5 wherein the locality parameter comprises a stream count comprising an incremented count of read requests for a sequential data range

7. The apparatus of claim 6 wherein the time parameter comprises an aging indication of a time span in which said read requests have been issued

8. The apparatus of claim 7 wherein the non-requested read ahead data are transferred when both parameters fall within a selected threshold range.

9. The apparatus of claim 5 wherein the data structure comprises a RAID stripe comprising contiguous data on a selected storage device of a storage array.

10. The apparatus of claim 9 wherein the requested data comprises a first portion of said RAID stripe, and wherein the speculative non-requested read ahead data comprises another portion of said RAID stripe.

11. The apparatus of claim 10 wherein a stripe data descriptor (SDD) is maintained in conjunction with the cached requested data and non-requested read ahead data of the RAID stripe, the SDD comprising a stream count variable and a stream size variable associated with said data.

12. A data storage system comprising:

a cache memory; and

means for satisfying access requests for sequential data by switching both ways between streaming requested data and streaming non-requested data from a storage array to the cache memory.

13. A method comprising:

providing a data storage system having a RASC;

placing pending read requests for sequential data in the RASC;

switching, in response to data stored in the RASC, from caching speculative non-requested read ahead data to read behind stream locking of read requests in the RASC.

14. The method of claim 13 wherein the switching step is characterized by the RASC indicating that a particular read request has overrun a contemporaneous non-requested read ahead data.

15. The method of claim 14 wherein the switching step is characterized by comparing a logical block address (LBA) associated with the read request to an LBA associated with the contemporaneous non-requested read ahead data.

16. The method of claim 15 wherein the switching step is characterized by initiating read behind stream locking of requested data when the read request LBA is greater than the non-requested read ahead data LBA.

17. The method of claim 16 wherein the switching step is characterized by re-initiating caching of non-requested read ahead data when the read request LBA changes to being less than the non-requested read ahead data LBA.

18. The method of claim 13 further comprising a step of initiating caching of non-requested read ahead data in relation to a time parameter and a locality parameter associated with a data structure of which the requested data forms a part.

19. The method of claim 18 wherein the initiating step is characterized by comparing a stream count defining an incremented count of read requests in a sequential data range with a selected threshold.

20. The method of claim 19 wherein the initiating step is characterized by maintaining a SDD in conjunction with the cached requested data and non-requested read ahead data, the SDD comprising the stream count associated with said data.