A Dialogue on Persistence

Part III 💬 Dialogue OSTEP pp. 417–418 · ~3 min read

The third pillar begins the way the others did — with the professor, the student, and (the student insists) a peach.

Professor:And thus we reach the third of our four… err… three pillars of operating systems: persistence .
Student:Did you say three pillars, or four? What’s the fourth?
Professor:No. Just three, young student, just three. Trying to keep it simple here.
Student:OK, fine. But what is persistence, oh fine and noble professor?
Professor:You know it in the traditional sense already — as the dictionary says: “a firm or obstinate continuance in a course of action in spite of difficulty or opposition.”
Student:Kind of like taking your class: some obstinance required.
Professor:Ha! Yes. But here it means something else. Imagine you’re in a field, and you pick a —
Student:(interrupting) I know! A peach! From a peach tree!
Professor:I was going to say an apple. Oh well; we’ll do it your way. You pick many peaches, and you want them to last a long time — winter is hard and cruel in Wisconsin. What do you do?
Student:Lots of things! Pickle it, bake a pie, make jam. Fun!
Professor:Fun? Maybe. But certainly a lot more work to make the peach persist. And so it is with information: making it persist — despite computer crashes, disk failures, and power outages — is a tough and interesting challenge.

The peach, and the point: raw things perish; preserved things persist — and the same is true of your data.

the peach🍑fresh → rots by winter🫙🥧jam / pie / picklepreserved → lastsextra work buys survival→your datain memory onlycrash / power loss → goneon disk / FS 💾persists across crashesI/O devices, disks, file systems, journaling — the work of Part III
Student:Nice segue; you’re getting quite good at that.
Professor:Thanks! A professor can always use a few kind words.
Student:I’ll try to remember that. Time to stop talking peaches and start talking computers?
Professor:Yes, it is that time…

The road through Part III

Persistence is a whole stack, and we’ll climb it from the metal up:

I/O devicesch36›HDD · SSDch37·44›RAIDch38›File systemsch39–41›Journaling · LFSch42–43›Data integritych45›Distributed FSch47–50from the raw device up to file systems that survive crashes and span networks

Check yourself: what persistence means

1.In the systems sense, what does persistence mean?

2.How does preserving a peach (jam, pie, pickle) map to the systems idea?

3.Why is achieving persistence a genuinely hard challenge (not just 'write the data down')?

4.Persistence is which of the OS's pillars, and how many are there in this book?

4 questions