ARIES data structures

1. ARIES data structures • Log sequence number (LSN) identifies each log record • Must be sequentially increasing • Each page contains a PageLSN which is the LSN of the last log record whose effects are reflected on the page • PageLSNis used during recovery to prevent repeated redo.

2. Cont. • Each log record contains LSN of previous log record of the same transaction • Special redo-only log record called compensation log record (CLR) used to log actions taken during recovery that never need to be undone • Have a field UndoNextLSN to note next (earlier) record to be undone (next operation to undo). • Required to avoid repeated undo of already undone actions

3. Log records contain following fields

5. Transaction Table • During recovery: • initialized during analysis pass from most recent checkpoint • modified during analysis as log records are encountered, and during undo

6. Cont. • DirtyPageTable • List of pages in the buffer that have been updated • Contains, for each such page • PageLSN of the page • RecLSNis an LSN such that log records before this LSN have already been applied to the page version on disk

7. ARIES Recovery Algorithm ARIES recovery involves three passes • Analysis pass: Determines • Which transactions to undo • Which pages were dirty (disk version not up to date) at time of crash • RedoLSN: LSN from which redo should start • Redo pass: • Repeats history, redoing all actions from RedoLSN • RecLSN and PageLSNs are used to avoid redoing actions already reflected on page • Undo pass: • Rolls back all incomplete transactions

8. ARIES Recovery: Analysis Analysis pass • Starts from last complete checkpoint log record • Reads in DirtyPageTable from log record • Sets RedoLSN = min of RecLSNs of all pages in DirtyPageTable • In case no pages are dirty, RedoLSN = checkpoint record’s LSN • Sets undo-list = list of transactions in checkpoint log record • Reads LSN of last log record for each transaction in undo-list from checkpoint log record • Scans forward from checkpoint

9. Cont. Analysis pass (cont.) • Scans forward from checkpoint • If any log record found for transaction not in undo-list, adds transaction to undo-list • Whenever an update log record is found • If page is not in DirtyPageTable, it is added with RecLSN set to LSN of the update log record • If transaction end log record found, delete transaction from undo-list • At end of analysis pass: • RedoLSN determines where to start redo pass • All transactions in undo-list need to be rolled back

10. ARIES Redo Pass Redo Pass: Repeats history by replaying every action not already reflected in the page on disk, as follows: • Scans forward from RedoLSN. Whenever an update log record is found: • If the page is not in DirtyPageTable or the LSN of the log record is less than the RecLSN of the page in DirtyPageTable, then skip the log record • Otherwise fetch the page from disk. If the PageLSN of the page fetched from disk is less than the LSN of the log record, redo the log record

11. ARIES: Undo Pass Undo pass • Performs backward scan on log undoing all transaction in undo-list • Backward scan optimized by skipping unneeded log records as follows: • Next LSN to be undone for each transaction set to LSN of last log record for transaction found by analysis pass. • At each step pick largest of these LSNs to undo, skip back to it and undo it • After undoing a log record • For ordinary log records, set next LSN to be undone for transaction to PrevLSN noted in the log record • For compensation log records (CLRs) set next LSN to be undo to UndoNextLSN noted in the log record • All intervening records are skipped since they would have been undo

12. Example 1 • After a crash, we find the following log: Analysis phase: Scan forward through the log starting at LSN 0.LSN 5: Initialize XACT table and DPT to empty.LSN 10: Add (T1, LSN 10) to XACT table. Add (P1, LSN 10) to DPT.LSN 15: Set LastLSN=15 for T1 in XACT table. Add (P2, LSN 15) to DPT.LSN 20: Change T1 status to "Commit" in XACT table

13. Redo phase: Scan forward through the log starting at LSN 10.LSN 10: Read page P1, check PageLSN (LSN of the most recent log record that describes a change to this page) stored in the page. If PageLSN<10, redo LSN 10 (set value to ZZZ) and set the page's PageLSN=10.LSN 15: Read page P2, check PageLSN stored in the page. If PageLSN<15, redo LSN 15 (set value to XXX) and set the page's PageLSN=15. Undo phase: Do nothing; no transactions to undo.

14. Example 2 • After a crash, we find the following log:

15. Analysis phase: Scan forward through the log starting at LSN 0.LSN 5: Initialize XACT table and DPT to empty.LSN 10: Add (T1, LSN 10) to XACT table. Add (P1, LSN 10) to DPT.LSN 15: Set LastLSN=15 for T1 in XACT table. Add (P2, LSN 15) to DPT.LSN 20: Add (T2, LSN 20) to XACT table. Add (P3, LSN 20) to DPT.LSN 25: Change T1 status to "Commit" in XACT tableLSN 30: Set LastLSN=30 for T2 in XACT table.

16. Redo phase: Scan forward through the log starting at LSN 10.LSN 10: Read page P1, check PageLSN stored in the page. If PageLSN<10, redo LSN 10 (set value to ZZZ) and set the page's PageLSN=10.LSN 15: Read page P2, check PageLSN stored in the page. If PageLSN<15, redo LSN 15 (set value to XXX) and set the page's PageLSN=15.LSN 20: Read page P3, check PageLSN stored in the page. If PageLSN<20, redo LSN 20 (set value to VVV) and set the page's PageLSN=20.LSN 30: Read page P1, check PageLSN stored in the page. It will be 10. Redo LSN 30 (set value to TTT) and set the page's PageLSN=30.

17. Undo phase: T2 must be undone. Put LSN 30 in ToUndo.Write Abort record to log for T2LSN 30: Undo LSN 30 - write a CLR for P1 with "set P1=ZZZ" and undonextLSN=20. LSN 20: Undo LSN 20 - write a CLR for P3 with "set P3=UUU" and undonextLSN=NULL. Write UUU into P3.