Mirror resynchnronization of fixed page length tables for better repair time to high availability in databases
In a mirrored database system, mirror resynchronization of fixed length pages is optimized for quick repair and high availability by identifying those lost page changes that must be made for resynchronization and ignoring other duplicate changes. Essential pages are identified using log sequence numbers (LSNs) to locate the most current version of a changed page, and other lower valued LSNs of less current versions of that page are eliminated as having changes that are duplicated in the current version of the page. Page changes since the last database checkpoint before losing mirror synchronization and the current LSN of the transaction log are identified as possibly necessary changes to be made.
1. A method of mirror resynchronization of pages in a mirrored pair of databases comprising a primary database and a mirror database, the method comprising:
writing, for each page change of a plurality of page changes to the pages, a record of the page change to a transaction log buffer, wherein each record includes a log sequence number (LSN);
maintaining a record of a greatest checkpoint LSN, the greatest checkpoint LSN being the greatest LSN of the LSNs of records from the transaction log buffer written to a primary transaction log file for the primary database and a mirror transaction log file for the mirror database;
upon a loss of mirror synchronization and until a starting of mirror resynchronization, capturing, in a change tracking log, page changes written to the transaction log buffer with an LSN greater than the greatest checkpoint LSN, while continuing writing a record of page changes to the transaction log buffer; and
after the starting of mirror resynchronization:
determining, for each page referenced by a page change in the change tracking log, a most recent page change for that page captured in the change tracking log, and
updating the mirror database using the most recent page changes in the change tracking log for each page referenced by a page change in the change tracking log.
2. The method of claim 1 , wherein continuing writing a record of page changes to the transaction log buffer comprises writing the record in parallel with updating the mirror database.
3. The method of claim 1 , wherein capturing page changes in the change tracking log comprises recording page numbers and log sequence numbers (LSNs) of new and changed pages into the change tracking log.
4. The method of claim 3 , wherein determining the most recent page changes for each page referenced by a page change in the change tracking log comprises sorting the page changes in the change tracking log by page number.
5. The method of claim 3 , wherein determining the most recent page changes for each page referenced by a page change in the change tracking log comprises finding, for each page referenced by a page change in the change tracking log, the page change having a largest LSN of the page changes referencing the page.
6. The method of claim 1 , further comprising flushing each resynchronized page change of a plurality of resynchronized page changes to the mirror database upon finishing mirror resynchronization.
7. A system for mirror resynchronization of pages in a mirrored pair of databases comprising a primary database and a mirror database, the system comprising:
one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising:
writing, for each page change of a plurality of page changes to the pages, a record of the page change to a transaction log buffer, wherein each record includes a log sequence number (LSN);
maintaining a record of a greatest checkpoint LSN, the greatest checkpoint LSN being the greatest LSN of the LSNs of records from the transaction log buffer written to a primary transaction log file for the primary database and a mirror transaction log file for the mirror database;
upon a loss of mirror synchronization and until a starting of mirror resynchronization, capturing, in a change tracking log, page changes written to the transaction log buffer with an LSN greater than the greatest checkpoint LSN, while continuing writing a record of page changes to the transaction log buffer; and
after the starting of mirror resynchronization:
determining, for each page referenced by a page change in the change tracking log, a most recent page change for that page captured in the change tracking log, and
updating the mirror database using the most recent page changes in the change tracking log for each page referenced by a page change in the change tracking log.
8. The system of claim 7 , wherein continuing writing a record of page changes to the transaction log buffer comprises writing the record in parallel with updating the mirror database.
9. The system of claim 7 , wherein capturing page changes in the change tracking log comprises recording page numbers and log sequence numbers (LSNs) of new and changed pages into the change tracking log.
10. The system of claim 9 , wherein determining the most recent page changes for each page referenced by a page change in the change tracking log comprises sorting the page changes in the change tracking log by page number.
11. The system of claim 9 , wherein determining the most recent page changes for each page referenced by a page change in the change tracking log comprises finding, for each page referenced by a page change in the change tracking log, the page change having a largest LSN of the page changes referencing the page.
12. The system of claim 7 , the operations further comprising flushing each resynchronized page change of a plurality of resynchronized page changes to the mirror database upon finishing mirror resynchronization.
13. A non-transitory computer readable storage medium comprising executable instructions for controlling the operation of a computer system to perform mirror resynchronization of fixed length pages in a mirrored pair of databases comprising a primary database and a mirror database supporting transactions, comprising instructions to perform operations:
writing, for each page change of a plurality of page changes to the pages, a record of the page change to a transaction log buffer, wherein each record includes a log sequence number (LSN);
maintaining a record of a greatest checkpoint LSN, the greatest checkpoint LSN being the greatest LSN of the LSNs of records from transaction log buffer written to a primary transaction log file for the primary database and a mirror transaction log file for the mirror database;
upon a loss of mirror synchronization and until a starting of mirror resynchronization, capturing, in a change tracking log, page changes written to the transaction log buffer with an LSN greater than the greatest checkpoint LSN, while continuing writing a record of page changes to the transaction log buffer; and
after the starting of mirror resynchronization:
determining, for each page referenced by a page change in the change tracking log, a most recent page change for that page captured in the change tracking log, and
updating the mirror database using the most recent page changes in the change tracking log for each page referenced by a page change in the change tracking log.
14. The non-transitory computer readable storage medium of claim 13 , wherein continuing writing a record of page changes to the transaction log buffer comprises writing the record in parallel with updating the mirror database.
15. The non-transitory computer readable storage medium of claim 13 , wherein capturing page changes in the change tracking log comprises recording page numbers and log sequence numbers (LSNs) of new and changed pages into the change tracking log.
16. The non-transitory computer readable storage medium of claim 15 , wherein determining the most recent page changes for each page referenced by a page change in the change tracking log comprises sorting the page changes in the change tracking log by page number.
17. The non-transitory computer readable storage medium of claim 15 , wherein determining the most recent page changes for each page referenced by a page change in the change tracking log comprises finding, for each page referenced by a page change in the change tracking log, the page change having a largest LSN of the page changes referencing the page.
18. The non-transitory computer readable storage medium of claim 13 , the operations further comprising flushing each resynchronized page change of a plurality of resynchronized page changes to the mirror database upon finishing mirror resynchronization.