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SubjectRe: [OFFTOPIC] Re: disk head scheduling
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"Richard B. Johnson" <root@chaos.analogic.com> writes:
> [snip]
> Expensive disk drives now do full track buffering. This costs
> money because RAM costs money. To buffer one full track on a
> Disc drive requires CAPACITY / (HEADS * CYLINDERS) which can
> be upwards of 100 megabytes of high-speed SRAM. Sector buffering
> is always necessary. It is part of the de-serializer and is
> required because the Disc internals are never synchronous with
> the outside world.

Why SRAM? The nice thing about SRAM is its low latencies. If you're
slurping a whole track off a disk, latencies shouldn't be an issue;
you might need lots of bandwidth, but you can get high bandwidth from
DRAM.

Besides, 100 megabytes of SRAM is 100*(8 bits in a byte)*(6
transistors for an SRAM cell) = 2.4 billion transistors! Ignoring
issues of cost, where exactly do they put all those chips? I don't
know much about the semiconductor state-of-the-art, but I do know that
at current levels of integration it would be a squeeze to fit that lot
on the back of a disk.

> [snip]
> In conclusion, all this bandwidth will be wasted in a year or
> two anyway. Disk drives currently under test use static RAM.
> This is not the Slow........ NVRAM where you write then read-read-
> read-read, etc., until it finally "takes". This is real static
> RAM with a 3-volt battery to keep it alive for 20 years. These
> drives use standard interfaces so you can just put them in your
> PC and be done with it.
> [snip]

Static RAM? As in the stuff that L2 caches are made out of? As in the
stuff that currently costs several times as DRAM for the same
capacity?

If static RAM can be manufactured economically in disk-sized lumps,
then WTF is the point of putting it at the other side of a PCI bus (or
any other IO bus for that matter) from the processor. I'd have N
gigabytes of the stuff as my main memory, thank you very much (having
it battery-backed-up would be the icing on the cake).

Dave Wragg

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