WAL & the Log
Each log record has a unique Log Sequence Number (LSN).
|LSNs always increasing.
Each data page contains a pageLSN.
|The LSN of the most recent log record for an update to that page.
System keeps track of flushedLSN.
|The max LSN flushed so far.
WAL: Before a page is written,
|pageLSN £ flushedLSN|