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

LSNs

pageLSNs

RAM

flushedLSN

Log records
flushed to disk

“Log tail”
in RAM

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