1. © Dennis Shasha, Philippe Bonnet 2001 Log Tuning

2. overview

3. Checkpointing Periodically, the DBMS creates a checkpoint, in order to minimize the time taken to recover in the event of a system crash. Write to log: – begin_checkpoint record: Indicates when chkpt began. –end_checkpoint record: Contains current Xact table and dirty page table. This is a `fuzzy checkpoint’: Other Xacts continue to run; so these tables accurate only as of the time of the begin_checkpoint record. No attempt to force dirty pages to disk; effectiveness of checkpoint limited by oldest unwritten change to a dirty page. (So it’s a good idea to periodically flush dirty pages to disk!) Store LSN of chkpt record in a safe place (master record). © Dennis Shasha, Philippe Bonnet 2001

4. The Big Picture: What’s Stored Where © Dennis Shasha, Philippe Bonnet 2001

5. Crash Recovery: Big Picture Start from a checkpoint (found via master record). Three phases. Need to: –Figure out which Xacts committed since checkpoint, which failed (Analysis). –REDO all actions. (repeat history) –UNDO effects of failed Xacts. © Dennis Shasha, Philippe Bonnet 2001

6. Aries for steal, no-force approach There are 3 phases in the Aries recovery algorithm: –Analysis: Scan the log forward (from the most recent checkpoint) to identify all Xacts that were active, and all dirty pages in the buffer pool at the time of the crash. –Redo: Redoes all updates to dirty pages in the buffer pool, as needed, to ensure that all logged updates are in fact carried out and written to disk. –Undo: The writes of all Xacts that were active at the crash are undone (by restoring the before value of the update, which is in the log record for the update), working backwards in the log. (Some care must be taken to handle the case of a crash occurring during the recovery process!) © Dennis Shasha, Philippe Bonnet 2001

7. example Get master record from disk, which is LSN 00, i.e., the begin checkpoint record. © Dennis Shasha, Philippe Bonnet 2001

8. Analysis T. T.: (T1,80),and (T3,60). D.P.T.: (P1,20), (P2,30), (P3,40), and (P5,80). © Dennis Shasha, Philippe Bonnet 2001

9. Redo Redo starts from LSN20 (minimum recLSN in DPT). –LSN 20 Check whether P1 has pageLSN more than 10 or not. Since it is a committed transaction, we probably need not redo this update. –LSN 30 Redo the change in P2 –LSN 40 Redo the change in P3 –LSN 50 No action –LSN 60 Redo the changes on P2 –LSN 70 No action –LSN 80 Redo the changes on P5 –LSN 90 No action © Dennis Shasha, Philippe Bonnet 2001

10. Undo ToUndo consists of (80, 60). LSN 80 – Undo the changes in P5. Append a CLR: Undo T1 LSN 80, set undonextLSN = 20. Add 20 to ToUndo. ToUndo consists of (60, 20). LSN 60 –Undo the changes on P2. Append a CLR: Undo T3 LSN 60, set undonextLSN = 40. Add 40 to ToUndo. ToUndo consists of (40, 20). LSN 40 –Undo the changes on P3. Append a CLR: Undo T3 LSN 40, T3 end ToUndo consists of (20). LSN 20 –Undo the changes on P1. Append a CLR: Undo T1 LSN 20, T1 end © Dennis Shasha, Philippe Bonnet 2001

11. The after recovery © Dennis Shasha, Philippe Bonnet 2001

12. summary