Recovery

Redo Winner Writes

Redo re-applies committed winner writes.

highlighted = computed this step

Redo winner writes

Redo re-applies writes from transactions that committed before the crash. In this surface, redo is winners-authoritative: committed writes define the recovered value for their items. Note: loser writes are not in the redo list.

redo winners\text{redo winners}

Redo list

The compiled recovery has 1 winner and 1 redo write. Note: the redo LSN list is rendered.

winners=1,redo=1\text{winners}=1,\quad \text{redo}=1

a simplified redo/undo recovery over a tiny log; ARIES detail (fuzzy checkpoints, CLRs, media recovery) and real durability/fsync are beyond these traces - no product claims.

Redo winner writes lsn txn type item before->after 1 T1 begin 2 T1 write A 0->5 3 T1 commit 4 T2 begin 5 T2 write B 0->9 CRASH winners: T1 losers: T2 recovered state item value A 5 B 0 redo LSNs: 2 undo LSNs: 5

Summary

Redo preserves committed work across the crash. Note: this is simplified redo and undo; ARIES detail, fuzzy checkpoints, CLRs, media recovery, and real fsync durability are deferred.

redo committed work\text{redo committed work}