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

Log records

flushed to disk

“Log tail”

in RAM

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