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Summary Dialogue on Persistence
Student: Wow, file systems seem interesting(!), and yet complicated.
Professor: That’s why my spouse and I do our research in this space.
Student: Hold on. Are you one of the professors who wrote this book? I thought
we were both just fake constructs, used to summarize some main points, and
perhaps add a little levity in the study of operating systems.
Professor: Uh… er… maybe. And none of your business! And who did you
think was writing these things? (sighs) Anyhow, let’s get on with it: what did
you learn?
Student: Well, I think I got one of the main points, which is that it is much
harder to manage data for a long time (persistently) than it is to manage data
that isn’t persistent (like the stuff in memory). After all, if your machines crashes,
memory contents disappear! But the stuff in the file system needs to live forever.
Professor: Well, as my friend Kevin Hultquist used to say, “Forever is a long
time”; while he was talking about plastic golf tees, it’s especially true for the
garbage that is found in most file systems.
Student: Well, you know what I mean! For a long time at least. And even simple
things, such as updating a persistent storage device, are complicated, because you
have to care what happens if you crash. Recovery, something I had never even
thought of when we were virtualizing memory, is now a big deal!
Professor: Too true. Updates to persistent storage have always been, and re-
main, a fun and challenging problem.
Student: I also learned about cool things like disk scheduling, and about data
protection techniques like RAID and even checksums. That stuff is cool.
Professor: I like those topics too. Though, if you really get into it, they can get
a little mathematical. Check out some of the latest on erasure codes if you want
your brain to hurt.
Student: I’ll get right on that.
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2 SUMMARY DIALOGUE ON PERSISTENCE
Professor: (frowns) I think you’re being sarcastic. Well, what else did you like?
Student: And I also liked all the thought that has gone into building technology-
aware systems, like FFS and LFS. Neat stuff! Being disk aware seems cool. But
will it matter anymore, with Flash and all the newest, latest technologies?
Professor: Good question! And a reminder to get working on that Flash chap-
ter… (scribbles note down to self) … But yes, even with Flash, all of this stuff
is still relevant, amazingly. For example, Flash Translation Layers (FTLs) use
log-structuring internally, to improve performance and reliability of Flash-based
SSDs. And thinking about locality is always useful. So while the technology
may be changing, many of the ideas we have studied will continue to be useful,
for a while at least.
Student: That’s good. I just spent all this time learning it, and I didn’t want it
to all be for no reason!
Professor: Professors wouldn’t do that to you, would they?
OPERATING
SYSTEMS
[VERSION 1.01]
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