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notes10_small - CPS216: Data-intensive Computing Systems...

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1 CPS216: Data-intensive Computing Systems Failure Recovery Shivnath Babu
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2 Key problem Unfinished transaction Example Constraint: A=B T 1 : A A 2 B B 2
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3 Examples: • Power goes off • Software bugs • Disk data is lost • Memory lost without CPU halt • CPU misbehaves (overheating) Unexpected Events:
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4 Storage hierarchy Memory Disk x x
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5 Operations: • Input (x): block containing x memory • Output (x): block containing x disk • Read (x,t): do input(x) if necessary t value of x in block • Write (x,t): do input(x) if necessary value of x in block t
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6 Key problem Unfinished transaction Example Constraint: A=B T 1 : A A 2 B B 2
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7 T 1 : Read (A,t); t t 2 Write (A,t); Read (B,t); t t 2 Write (B,t); Output (A); Output (B); A: 8 B: 8 A: 8 B: 8 memory disk 16 16 16 failure!
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8 • Need atomicity: execute all actions of a transaction or none at all
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9 One solution: undo logging (immediate modification) due to: Hansel and Gretel, 782 AD
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10 T 1 : Read (A,t); t t 2 A=B Write (A,t); Read (B,t); t t 2 Write (B,t); Output (A); Output (B); A:8 B:8 A:8 B:8 memory disk log Undo logging (Immediate modification) 16 16 <T1, start> <T1, A, 8> <T1, commit> 16 <T1, B, 8> 16
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11 One “complication” • Log is first written in memory • Not written to disk on every action memory DB Log A: 8 16 B: 8 16 Log: <T 1 ,start> <T 1 , A, 8> <T 1 , B, 8> A: 8 B: 8 16 BAD STATE # 1
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12 One “complication” • Log is first written in memory • Not written to disk on every action memory DB Log A: 8 16 B: 8 16 Log: <T 1 ,start> <T 1 , A, 8> <T 1 , B, 8> <T 1 , commit> A: 8 B: 8 16 BAD STATE # 2 <T1, B, 8> <T1, commit> ...
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Undo logging rules (1) For every action generate undo log record (containing old value) (2) Before x is modified on disk, log records pertaining to x must be on disk (write ahead logging: WAL) (3) Before commit is flushed to log, all writes of transaction must be reflected on disk
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This document was uploaded on 01/17/2012.

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notes10_small - CPS216: Data-intensive Computing Systems...

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