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notes10_small - CPS216: Dataintensive 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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13 Undo logging rules (1) For every action generate undo log record (containing old value) (2) Before 
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notes10_small - CPS216: Dataintensive Computing Systems...

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