Write-Ahead Logging

Describe every change in a log before making it, force the log at commit, and a crash can never lose a committed transaction or keep an uncommitted one.

Log before data

Steal / no-force

Redo then undo

Workload

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Two Rules, Two Policies

Write-ahead

A page reaches disk only after the log through its pageLSN does.

Force at commit

COMMIT returns only once its log record is on disk.

Steal

Dirty pages of running transactions may be written — so recovery needs undo.

No-force

Committed pages need not be written at commit — so recovery needs redo.

Real systems also write checkpoints, so analysis and redo start from a recent point instead of the beginning of the log.

Knowledge Check

Question 1 of 3

What exactly does the write-ahead rule require?

DSAverse

Write-Ahead Logging

Two Rules, Two Policies

Knowledge Check

See also

Write-Ahead Logging

Two Rules, Two Policies

Knowledge Check

See also