Maintaining data consistency in mirrored cluster storage systems with write-back cache

• US 7,908,448 B1
• Filed: 01/11/2008
• Issued: 03/15/2011
• Est. Priority Date: 01/30/2007
• Status: Active Grant

• Current Assignee: AmZetta Technologies, LLC (American Megatrends International, LLC)
• Original Assignee: American Megatrends Incorporated
• Inventors: Jayaraman, Raja, Chatterjee, Paresh, Balakrishnan, Narayanan
• Primary Examiner(s): Kim; Matt
• Assistant Examiner(s): Dudek, Jr.; Edward J
• Application Number:
  US12/013,116
• Time in Patent Office: 1,159 Days
• Field of Search: 711/162, 711/135, 711/E12.103, 707/655
• US Class Current: 711/162
• CPC Class Codes: G06F 11/2064 while ensuring consistency
• CPC Class Codes: G06F 11/2082 Data synchronisation

Abstract

Techniques for maintaining mirrored storage cluster data consistency on systems with write back cache can employ write-intent logging and periodic log clearing. The techniques can track any outstanding I/Os, data in caches, and data that has gone out of sync between mirrored nodes due to link failures. The techniques can ensure that a power failure on any of the storage nodes does not result in inconsistent data among the storage nodes. The techniques can use gate bitmaps to log outstanding I/Os. The gate bitmaps may be periodically cleared. The clearing period may be selected as to allow enough time for all caches to be flushed and for all synchronizing I/Os to complete. Efficient queuing of I/O requests in conjunction with the efficient operation of multiple gate bitmaps can reduce performance impacts of the gating or logging operations.

Claims

1. A method for maintaining mirrored data consistency between a primary networked storage node and a secondary networked storage node using write-intent logging, the method comprising:

establishing a delay period;

logging, on the primary networked storage node, an intent to write data, comprising setting a bit in a gate bitmap, the bit corresponding to a storage area within the primary networked storage node where the data is to be written;

writing the data to the primary networked storage node after logging the intent to write data;

relaying a request to the secondary networked storage node to minor the data after logging the intent to write data; and

after the delay period completes, clearing the bit in the gate bitmap to protect an underlying write-back cache, wherein any entries of the write-back cache made before the delay period began are flushed to a disk prior to the delay period completing.

2. The method of claim 1, wherein logging further comprises storing the gate bitmap to the disk.

3. The method of claim 1, wherein the delay period is longer than a specified time for the underlying write-back cache to flush.

4. The method of claim 1, wherein the delay period is longer than a specified time for relaying the request to the secondary networked storage node.

5. The method of claim 1, further comprising providing a delta tab in the primary networked storage node to temporarily store a difference between data on the primary networked storage node and data on the secondary networked storage node.

6. A computer storage medium having computer-executable instructions stored thereon which, when executed by a computer system, cause the computer system to:

establish a delay period;

receive an input/output (I/O) request;

record the I/O request in a log comprising a gate bitmap, bits of the gate bitmap corresponding to respective storage areas of a disk;

store the log to the disk;

execute the I/O request after the log has been stored to disk;

relay the I/O request to a mirrored computer system for execution;

wait for the delay period to complete; and

clear the log after the wait by clearing the bits of the gate bitmap to protect an underlying write-back cache, wherein any entries of the write-back cache made before the delay period began are flushed prior to the delay period completing.

7. The computer storage medium of claim 6, wherein the delay period is longer than a specified time for flushing the underlying write-back cache.

8. The computer storage medium of claim 6, wherein the delay period is longer than a specified time for relaying the I/O request to the mirrored computer system.

9. The computer storage medium of claim 6, wherein storing the log to the disk comprises flushing a disk cache.

10. The computer storage medium of claim 6, further comprising providing a delta tab to temporarily accumulate differences between data on the computer system and data on the mirrored computer system.

11. The computer storage medium of claim 6, wherein relaying the I/O request to the mirrored computer system comprises logging the I/O request in a delta tab until the relayed I/O request is completed.

12. The computer storage medium of claim 10, wherein clearing the log excludes clearing entries pending in the delta tab.

13. A method for maintaining mirrored data consistency between a primary networked storage node and a secondary networked storage node using write-intent logging, the method comprising:

establishing a delay period;

receiving an input/output (I/O) request at the primary networked storage node, comprising;

evaluating a log entry within a flush gate bitmap associated with an area of a disk targeted by the I/O request;

evaluating a log entry within a committed gate bitmap associated with an area of the disk targeted by the I/O request;

placing the I/O request into a wait queue if the log entry within the flush gate bitmap is not set;

placing the I/O request into a hold queue if the log entry within the flush gate bitmap is set and the log entry within the committed gate bitmap is not set;

executing the I/O request if the log entry within the flush gate bitmap is set and the log entry within the committed gate bitmap is set; and

processing the wait queue and the hold queue;

logging the I/O request to a log at the primary networked storage node;

storing the log to a disk associated with the primary networked storage node;

executing the I/O request at the primary networked storage node;

relaying the I/O request to the secondary networked storage node for execution;

waiting for the established delay period; and

clearing the log after waiting.

14. The method of claim 13, wherein the log comprises a gate bitmap, bits of the gate bitmap corresponding to respective storage areas of the disk.

15. The method of claim 13, wherein processing the wait queue and the hold queue comprises:

retrieving the I/O request from the wait queue;

setting the log entry associated with an area of the disk targeted by the I/O request retrieved from the wait queue; and

placing the I/O request retrieved from the wait queue into the hold queue after setting the log entry.

16. The method of claim 13, wherein processing the wait queue and the hold queue comprises retrieving the I/O request from the hold queue prior to executing the I/O request at the primary networked storage node.