Messages in this thread | | | Date | 23 Mar 1999 19:02:00 +0200 | From | (Kai Henningsen) | Subject | Re: [OFFTOPIC] Re: disk head scheduling |
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root@chaos.analogic.com (Richard B. Johnson) wrote on 22.03.99 in <Pine.LNX.3.95.990322115933.475A-100000@chaos.analogic.com>:
> On 22 Mar 1999, David Wragg wrote: > > > "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. > > > > Because dynamic RAM needs refresh. Refresh takes power. You need > to keep the data without power, i.e., the bias from a small battery > that is essentially shelf-life.
And typical SRAM, at last whenever I've heard about it, needs *more* power per bit. SRAM electronics are quite a bit more complex per bit than DRAM electronics. That's also what makes it so expensive. One bit of DRAM is not much more than one capacitor. (And faster stuff is also typically more energy intensive.)
Of course, silicon disks wouldn't be made from typical SRAM. That would be silly. Instead, they'd be made either from DRAM and require permanent power, or be made from some sort of NVRAM like the "CMOS clock" chips, and just might be able to run from a battery.
However, I have my doubts that either NVRAM technology or battery technology is advanced enough to create affordable disks in the gigabyte range, and anything else would have a very small market.
> > 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?
Wasn't the usual rule 4x the price, 1/4th the capacity? :-)
MfG Kai
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