Durability
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A tomicity
C
I
D urability 

1. Any change by a committed transaction must be made durable.
2. Any changes by a rolled back transaction must be removed.

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Log: append only mechanism

LSN: log sequence number
Record: T1 update P1 A (old value) B (new value)


Write Ahead Logging (WAL)
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Before writing any data page that is updated from memory to disk

First, flush log to disk
If completed, then write the data to disk