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From: BGB <cr88192@gmail.com>
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Subject: Re: Is Parallel Programming Hard, And, If So, What Can You Do About
 It?
Date: Tue, 20 May 2025 12:42:38 -0500
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On 5/20/2025 9:49 AM, Stefan Monnier wrote:
>> You need RAM to make this work, and a few MB of HDD side cache isn't a huge
>> cost if one would have already needed the same stuff to make the HDD work.
> 
> Indeed, AFAIK, what we call "HDD cache" is actually just the RAM used
> by the embedded CPU inside the drive for its operation.
> I expect this is used to store the information about in-flight requests
> (e.g. most importantly the data about the write requests received but
> that haven't yet reached the platters), but I also expects it holds data
> that happened to fly recently by the read-head, just in case.
> 

Probably.

As I understand it, it is this, along with a certain amount of "read 
prefetch", which is granted, typically the data for the rest of the 
track as the drive spins around;
And, keeping some copies of previously read content around, which can be 
read again from this cache if they happen to be requested.

As I understand it, also more modern HDDs tend to be "density per area" 
rather than angular slices (as it was on much older HDDs and floppies, 
*), so there would be more sectors on outer tracks vs on inner tracks.



*: Though, IIRC, there were some oddball floppy drives / variants that 
did something similar, and could get significant improvements in 
capacity while still using the same (or similar) physical media 
(although, incompatible with more normal floppy drives).

Eg: apparently things like LS-120 drives with 32MB on a 3.5" floppy sort 
of things, vs more on its specialized disks.


Well, vs the 2.88MB format, which simply shoved more sectors onto the 
disk...

Most everything else reached 1.44MB and died on this hill.



> 
>          Stefan