One undo of a truncate coming right up...

CREATE TABLE TestingTruncate (SomeUselessColumn varchar(50))

INSERT INTO TestingTruncate

SELECT NAME FROM sys.columns

SELECT * FROM TestingTruncate -- 501 rows

BEGIN TRANSACTION

TRUNCATE TABLE TestingTruncate

SELECT * FROM TestingTruncate -- 0 rows

ROLLBACK TRANSACTION

SELECT * FROM TestingTruncate -- 501 rows

btw, Create and Drop table are fully undoable as well.

BEGIN TRANSACTION

DROP TABLE TestingTruncate

ROLLBACK TRANSACTION

SELECT * FROM TestingTruncate

A similar example with create table is left as an exercise to the reader.

If an operation didn't write log entries for undo, then if that operation was part of a transaction and the transaction failed, the entire database would have to be marked suspect.

If an operation didn't write log entries for redo then neither mirroring nor log shipping (nor transactional replication) would be able to repeat that operation at the mirror/secondary