redbookcover.gif (13955 bytes) Readings in Database Systems, 3rd Edition

Stonebraker & Hellerstein, eds.

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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1998, Joseph M. Hellerstein.  Last modified 08/17/98.
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