Exercise 2: Why a Corrupted Tuesday Incremental Breaks Perfectly Intact Later Files — Possible Solution ==================================================================== WHAT WEDNESDAY'S AND THURSDAY'S INCREMENTALS ACTUALLY DESCRIBE ------------------------------ Per this chapter, each incremental only contains "what changed since the previous day's job." Wednesday's incremental doesn't describe the full state of the data as of Wednesday - it only describes the difference between Tuesday's state and Wednesday's state. It has no meaning on its own; it only makes sense as an instruction to apply on top of whatever Tuesday's own restore produced. WHY "INTACT" DOESN'T MEAN "USABLE" HERE ------------------------------ Wednesday's incremental file being perfectly intact means the delta information it contains is correctly stored and readable - but that delta was always defined relative to Tuesday's end state. If Tuesday's incremental is missing or corrupted, there is no correct Tuesday end state to apply Wednesday's delta on top of. Wednesday's file being intact doesn't help, because the problem isn't with Wednesday's file itself - it's that the file it depends on is gone. WHY THIS IS A CHAIN, NOT A SET OF INDEPENDENT PIECES ------------------------------ Per this chapter's own warn-box, "the chain breaks at exactly the point of the bad link, taking everything after it down with it." Each incremental is only meaningful in the specific sequence Full → Mon → Tue → Wed → Thu; skipping a broken link the way you might skip a missing puzzle piece isn't possible, because every later piece is defined in terms of the exact state the previous piece was supposed to produce. WHY THE DIFFERENTIAL SCHEME DOESN'T SHARE THIS PROBLEM ------------------------------ A differential file is always measured against the same fixed full backup, not against the previous day - so a corrupted Tuesday differential has no bearing on whether Wednesday's or Thursday's differentials can be used; each stands on its own. WHY THIS WORKS AS AN ANSWER ------------------------------ It explains that Wednesday's and Thursday's incrementals are only meaningful relative to Tuesday's own restored state, explains why their own file integrity doesn't matter if that dependency is broken, and contrasts this with why the differential scheme doesn't have the same vulnerability.