ARIES (& Logging) April 2-4, 2018

Transcription

1 ARIES (& Logging) April 2-4,

2 What does it mean for a transaction to be committed? 2

3 If commit returns successfully, the transaction is recorded completely (atomicity) left the database in a stable state (consistency) s effects are independent of other xacts (isolation) will survive failures (durability) 3

4 commit returns successfully = the xact s effects are visible forever 4

5 commit returns successfully = the xact s effects are visible forever commit called but doesn t return = the xact s effects may be visible 5

6 Motivation T1 T2 T3 T4 T5 Time 6

7 Motivation T1 T2 T3 T4 T5 Time 6

8 Motivation T1 T2 T3 T4 T5 Time 6

9 Motivation T1 T2 T3 T4 T5 Time 6

10 Motivation T1 T2 T3 T4 T5 Time 6

11 Motivation T1 T2 T3 T4 T5 Time 6

12 Motivation T1 T2 T3 T4 T5 Time 6

13 Motivation T1 CRASH! T2 T3 T4 T5 Time 6 Image copyright: Wikimedia Commons

14 Motivation Committed Transactions. These should be present when the DB restarts. T1 CRASH! T2 T3 T4 T5 Time 6 Image copyright: Wikimedia Commons

15 Motivation Committed Transactions. These should be present when the DB restarts. T1 CRASH! T2 T3 T4 T5 Time 6 Uncommitted Transactions. These should leave no trace Image copyright: Wikimedia Commons

16 How do we guarantee durability under failures? How do aborted transactions get rolled back? How do we guarantee atomicity under failures? 7

17 Problem 1: Providing durability under failures. 8

18 Simplified Model When a write succeeds, the data is completely written 9

19 Problems A crash occurs part-way through the write. A crash occurs before buffered data is written. 10

20 Write-Ahead Logging Before writing to the database, first write what you plan to write to a log file Log W(A:10) A 8 B 12 C 5 D 18 E Image copyright: OpenClipart (rg1024)

21 Write-Ahead Logging Once the log is safely on disk you can write the database Log W(A:10) A 8/ 10 B 12 C 5 D 18 E Image copyright: OpenClipart (rg1024)

22 Write-Ahead Logging Log is append-only, so writes are always efficient Log W(A:10) W(C:8) W(E:9) A 8/ 10 B 12 C 5 D 18 E Image copyright: OpenClipart (rg1024)

23 Write-Ahead Logging allowing random writes to be safely batched Log W(A:10) W(C:8) W(E:9) A 8/ 10 B 12 C 5/ 8 D 18 E 16 / 9 14 Image copyright: OpenClipart (rg1024)

24 Problem 2: Providing rollback. 15

25 Single DB Model Txn 1 Txn 2 A = 20 E = 19 B = 14 B = 15 COMMIT ABORT A 8 B 12 C 5 D 18 E Image copyright: OpenClipart (rg1024)

26 Single DB Model Txn 1 Txn 2 A = 20 E = 19 B = 14 B = 15 COMMIT ABORT A 8/ 20 B 12 C 5 D 18 E Image copyright: OpenClipart (rg1024)

27 Single DB Model Txn 1 Txn 2 A = 20 E = 19 B = 14 B = 15 COMMIT ABORT A 8/ 20 B 12 C 5 / D 18 / E Image copyright: OpenClipart (rg1024)

28 Single DB Model Txn 1 Txn 2 A = 20 E = 19 B = 14 B = 15 COMMIT ABORT A 8/ 20 B 12 C 5 / 14 D 18 / E Image copyright: OpenClipart (rg1024)

29 Single DB Model Txn 1 Txn 2 A = 20 E = 19 B = 14 B = 15 COMMIT ABORT A 8/ 20 B 12 / C 5 D / 15 / E Image copyright: OpenClipart (rg1024)

30 Txn 1 A = 20 B = 14 COMMIT Staged DB Model Txn 2 E = 19 B = 15 ABORT A 8 B 12 C 5 D 18 E 16 A 8 B 12 C 5 D 18 E Image copyright: OpenClipart (rg1024)

31 Txn 1 A = 20 B = 14 COMMIT Staged DB Model Txn 2 A 8/ 20 A 8 E = 19 B = 15 ABORT B 12 C 5 / 14 B 12 C 5 / 15 D 18 D 18 E 16 / E Image copyright: OpenClipart (rg1024)

32 Txn 1 A = 20 B = 14 COMMIT Staged DB Model Txn 2 E = 19 B = 15 ABORT A 8/ 20 B 12 / C 5 D E Image copyright: OpenClipart (rg1024)

33 Is staging always possible? 24

34 Staging takes up more memory. Merging after-the-fact can be harder. Merging after-the-fact introduces more latency! 25

35 for the single database model Problem 2: Providing rollback. ^ 26

36 UNDO Logging Store both the old and the new values of the record being replaced Log W(A:8!10) W(C:5!8) W(E:16!9) A 8/ 10 B 12 C 5/ 8 D 18 E 16 / 9 27 Image copyright: OpenClipart (rg1024)

37 UNDO Logging A 8/ 10 B 12 Active Xacts Xact:1, Log: 45 43: 44: Xact:2, Log: 32 45: Log W(A:8!10) W(C:5!8) W(E:16!9) C 5/ 8 D 18 E 16 / 9 28 Image copyright: OpenClipart (rg1024)

38 UNDO Logging A 8/ 10 B 12 Active Xacts Xact:1, ABORT Log: 45 43: 44: Xact:2, Log: 32 45: Log W(A:8!10) W(C:5!8) W(E:16!9) C 5/ 8 D 18 E 16 / 9 29 Image copyright: OpenClipart (rg1024)

39 UNDO Logging A 8/ 10 B 12 Active Xacts Xact:1, ABORT Log: 45 43: 44: Xact:2, Log: 32 45: Log W(A:8!10) W(C:5!8) W(E:16!9) C 5/ 8 D 18 E Image copyright: OpenClipart (rg1024)

40 UNDO Logging A 8/ 10 B 12 Active Xacts Xact:1, ABORT Log: 45 43: 44: Xact:2, Log: 32 45: Log W(A:8!10) W(C:5!8) W(E:16!9) C 5 D 18 E Image copyright: OpenClipart (rg1024)

41 UNDO Logging A 8 B 12 Active Xacts Xact:1, ABORT Log: 45 43: 44: Xact:2, Log: 32 45: Log W(A:8!10) W(C:5!8) W(E:16!9) C 5 D 18 E Image copyright: OpenClipart (rg1024)

42 Log Sequence Number Linked Lists Transaction Table Log 33

43 Log Sequence Number Linked Lists Transaction Table ABORT [XID] Log (necessary for crash recovery) 33

44 Log Sequence Number Linked Lists Transaction Table XID, LastLSN ABORT [XID] Log (necessary for crash recovery) 33

45 Log Sequence Number Linked Lists Transaction Table XID, LastLSN LSN, Prev LSN, Prev Image, ABORT [XID] Log (necessary for crash recovery) 33

46 Log Sequence Number Linked Lists Transaction Table XID, LastLSN LSN, Prev LSN, Prev Image, ABORT [XID] Log (necessary for crash recovery) 33

47 Log Sequence Number Linked Lists Transaction Table XID, LastLSN LSN, Prev LSN, Prev Image, LSN, Prev LSN, Prev Image, ABORT [XID] Log (necessary for crash recovery) 33

48 Problem 3: Providing atomicity. 34

49 Goal: Be able to reconstruct all state at the time of the DB s crash (minus all running xacts) 35

50 What state is relevant? 36

51 DB State Active Xacts Xact:1, Log: 45 43: 44: Xact:2, Log: 32 45: Log W(A:8!10) W(C:5!8) W(E:16!9) A 8/ 10 B 12 C 5/ 8 D 18 E 16 / 9 37 Image copyright: OpenClipart (rg1024)

52 DB State On-Disk (or rebuildable) In-Memory Only! Active Xacts Xact:1, Log: 45 43: 44: Xact:2, Log: 32 45: On-Disk Log W(A:8!10) W(C:5!8) W(E:16!9) A 8/ 10 B 12 C 5/ 8 D 18 E 16 / 9 37 Image copyright: OpenClipart (rg1024)

53 Rebuilding the Xact Table Log every COMMIT (replay triggers commit process) Log every ABORT (replay triggers abort process) New message: END (replay removes Xact from Xact Table) 38

54 Rebuilding the Xact Table Log every COMMIT (replay triggers commit process) Log every ABORT (replay triggers abort process) New message: END (replay removes Xact from Xact Table) What about BEGIN? (when does an Xact get added to the Table?) 38

55 Transaction Commit Write Commit Record to Log All Log records up to the transaction s LastLSN are flushed. Note that Log Flushes are Sequential, Synchronous Writes to Disk Commit() returns. Write End record to log. 39

56 Simple Transaction Abort (supporting crash recovery) Before restoring the old value of a page, write a Compensation Log Record (CLR). Logging continues during UNDO processing. CLR has an extra field: UndoNextLSN Points to the next LSN to undo (the PrevLSN of the record currently being undone) CLRs are never UNDOne. But might be REDOne when repeating history. (Why?) 40

57 Rebuilding the Xact Table Optimization: Write the Xact Table to the log periodically. (checkpointing) 41

58 ARIES Crash Recovery Start from checkpoint stored in master record. Analysis: Rebuild the Xact Table Redo: Replay operations from all live Xacts (even uncommitted ones). Undo: Revert operations from all uncommitted/aborted Xacts. Oldest log record of transaction active at crash Smallest reclsn in dirty page table after Analysis Last Checkpoint CRASH A R U 42