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by Joker_vD 277 days ago
> disks are not considered "slow devices".

And neither are the tapes. But the pipes, apparently, are.

Well, unfortunately, disk^H^H^H^H large persistent storage I/O is actually slow, or people wouldn't have been writing thread-pools to make it look asynchrnous, or sometimes even process-pools to convert disk I/O to pipe I/O, for the last two decades.
2 comments
jstimpfle 277 days ago
There is a misunderstanding. "Slow device" in the POSIX sense is about unpredictability, not maximal possible bandwidth. Reading from a spinning disk might be comparably slow in the bandwidth sense, but it's actually quite deterministic how much data you can shovel to or from it.

A pipe on the other hand might easily stall for an hour. The kernel generally can't know how long it will have to wait for more data. That's why pipe reads (as well as writes) are interruptible.

The absolute bandwidth number of a harddisk doesn't matter --- in principle you can overload any system such that it fails to schedule and complete all processes in time. Putting aside possible system overload, the "slow device" terminology makes a lot of sense.

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Joker_vD 277 days ago
Seeking a tape also takes an unpredictable amount of time; and so is seeking a disk, for that matter (IIRC, historically it was actually quite difficult for UNIX systems to saturate disk's througput with random reads).
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jstimpfle 277 days ago
According to ChatGPT, a tape device is actually considered a "slow device". Even though I'm not sure it's that unpredictable. Maybe for most common use cases it is.

I was under the impression that seeking a disk you can generally calculate well with 10ms? Again, it depends on the file system abstractions built on top, and then the cache and the current system load -- how many seeks will be required?

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jcalvinowens 276 days ago
>> disks are not considered "slow devices".

> And neither are the tapes. But the pipes, apparently, are.

The "slow vs fast" language is really unfortunate, I realize it's traditional but it's unnecessarily confusing.

A better way to conceptualize it IMHO is bounded vs unbounded: a file or a tape contains a fixed amount of data known a priori, a socket or a pipe does not.

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