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Execution of Transactions (cont)

States as the transaction executes: 

t   Action        v  Buf(A)  Buf(B)  Disk(A)  Disk(B)
-----------------------------------------------------
(0) BEGIN         .      .       .        8        5
(1) READ(A,v)     8      8       .        8        5
(2) v = v*2      16      8       .        8        5
(3) WRITE(A,v)   16     16       .        8        5
(4) READ(B,v)     5     16       5        8        5
(5) v = v+1       6     16       5        8        5
(6) WRITE(B,v)    6     16       6        8        5
(7) OUTPUT(A)     6     16       6       16        5
(8) OUTPUT(B)     6     16       6       16        6

After tx completes, we must have either
Disk(A)=8, Disk(B)=5   or   Disk(A)=16, Disk(B)=6

If system crashes before (8), may need to undo disk changes.
If system crashes after (8), may need to redo disk changes.