Read ahead storage control

• US 7,996,623 B2
• Filed: 06/30/2006
• Issued: 08/09/2011
• Est. Priority Date: 06/30/2006
• Status: Expired due to Fees

• Current Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Original Assignee: Seagate Technology LLC (Seagate Technology Holdings Plc)
• Inventors: Walker, Michael D.
• Primary Examiner(s): Shah; Sanjiv
• Assistant Examiner(s): DILLON, SAMUEL A
• Application Number:
  US11/480,086
• Publication Number:
  US 20080005481A1
• Time in Patent Office: 1,866 Days
• Field of Search: 711/118, 711/137, 711/167, 711/169, 712/205, 712/207, 710/5, 710/6
• 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 storage processor executing read commands from a host to retrieve data stored in a storage space; and

a cache manager that identifies a read stream of two or more of the read commands that are for sequential data, that derives a quantitative value for the identified read stream in terms of stream block count and time, that in addition to streaming requested data from the storage space associated with the identified read stream also streams a speculative stream of an indeterminate number of blocks, indeterminate at the time the speculative stream begins, from the storage space when the quantitative value is greater than a predetermined threshold value until switching from streaming the speculative stream of the indeterminate number of blocks to streaming other requested data associated with subsequent read commands from the host based on a comparison of a storage location of requested data associated with one of the subsequent read commands from the host to a contemporaneous storage location of the speculative stream of the indeterminate number of blocks.

2. The apparatus of claim 1 wherein the comparison indicates that the subsequent one of the read commands from the host has overrun the contemporaneous speculative stream of the indeterminate number of blocks.

3. The apparatus of claim 2 wherein the comparison is that of a logical block address (LBA) associated with the subsequent one of the read commands from the host to an LBA associated with the contemporaneous speculative stream of the indeterminate number of blocks.

4. The apparatus of claim 3 wherein the cache manager switches from streaming the speculative stream of the indeterminate number of blocks to streaming the requested data for the read commands from the host when the subsequent one of the read commands LBA is greater than the contemporaneous speculative stream LBA of the indeterminate number of blocks.

11. The apparatus of claim 3 wherein the cache manager switches from streaming the speculative stream of the indeterminate number of blocks to streaming the requested data for the read commands from the host when the subsequent one of the read commands from the host LBA and the speculative stream LBA of the indeterminate number of blocks define a range that is greater than a predetermined threshold range.

5. A method comprising:

receiving read commands for data that are stored in a storage space;

identifying a read stream of two or more of the read commands that are for sequential data;

deriving a quantitative value for the identified read stream in terms of stream block count and time;

comparing the quantitative value to a predetermined threshold value;

when the comparing values step determines the quantitative value exceeds the predetermined threshold value, streaming a speculative stream of an indeterminate number of blocks, indeterminate at the time the speculative stream begins, from the storage space in addition to streaming the data associated with the identified read stream;

during the streaming the indeterminate number of blocks step, comparing a location of data associated with a subsequent read command from the host to a contemporaneous storage location of the streaming the speculative stream of the indeterminate number of blocks; and

when the comparing locations step satisfies a predefined criteria, switching from streaming the speculative stream of the indeterminate number of blocks to streaming data associated with other read commands from the host.

6. The method of claim 5 wherein the switching step occurs when the comparing locations step indicates that the subsequent one of the read commands from the host has overrun the contemporaneous storage location of the streaming the speculative stream of the indeterminate number of blocks.

7. The method of claim 6 wherein the comparing locations step is characterized by comparing a logical block address (LBA) associated with the subsequent one of the read commands from the host to an LBA associated with the contemporaneous storage location of the streaming the speculative stream of the indeterminate number of blocks.

8. The method of claim 7 wherein the switching step occurs when the subsequent one of the read commands from the host LBA is greater than the contemporaneous storage location LBA of the streaming the speculative stream of the indeterminate number of blocks.

9. The method of claim 8 comprising switching again to streaming another speculative stream of an indeterminate number of blocks when another subsequent one of the read commands from the host LBA is less than the contemporaneous storage location LBA of the streaming the speculative stream of the indeterminate number of blocks.

10. The method of claim 5 wherein the switching step occurs when the comparing locations step indicates that the subsequent one of the read commands from the host and the contemporaneous storage location of the streaming the speculative stream of the indeterminate number of blocks define a range that exceeds a predetermined threshold range.