rcd@ico.isc.com (Dick Dunn) (05/03/90)
pcg@cs.aber.ac.uk (Piercarlo Grandi) writes: [somebody else said...] > What is this? Different drives have DIFFERENT > speeds. Even the same supplier does NOT adhere to the same speeds. > > Bah. Virtually every drive around does 3600 RPM,... This is true nowadays, on PC-class drives, but... >...This value of 3600 RPM has been constant for the past 30 years. from > mainframes to micros, and for good (e.g. mechanical) cause... Foo, not a chance. It's gotten more constant as time has gone by, but in the past it was a lot more common to see slower drives. The RK05 and RL01, among other unfortunately common drives, were both in the low-to-mid 2000's. The slowest drive I recall was 960 rpm (and alas, rotational latency was not the limiting factor). In fact, the trend has been *toward* 3600 rpm as a standard in the past decade or so, for causes as much electromagnetic as mechanical: You're not just interested in latency; you care about capacity. If you want to increase the rotational speed without losing capacity, you have to increase the linear recording density. As recording technology has improved (better coatings, plated media, thin-film heads, vertical recording) the bit densities have gone up and the drives have gotten smaller, with the rotation speed tending to become a constant. In the past there have been very strong constraints on the transfer rate systems could handle. The old IBM mainframe 1.5 Mb/s channel limit held for what seemed like ages relative to speed improvements in the other parts of systems. >...There have > been steady improvements in seek times and bandwidth, and rotational > latency is thus becoming the bottleneck... Yes, but this has happened relatively slowly. CPU speeds double every year; seek times might be halved every ten years. We're finally getting down to where the rotational latency is obnoxious enough that there's an incentive to spin faster. But it's not getting as bad as you might think as fast as you might think. Rotational latency is most obnoxious on track switch and single-cylinder seeks, but you can compensate reasonably by sensible allocation and skewing. Track-caching controllers help too. >...Palliatives revolve around the > multiple arm idea, i.e. reinventing drums, e.g. disc arrays (introduced > to the 386 world by compaq or zenith recently)... Multiple arms have been around for a while in the mainframe world, but don't expect to see them in this part of the world for quite a while. They're bulkier, require more peak power, are more expensive to build (you need two of not just actuator but all the servo and r/w systems), and are electromechanically tricky because one actuator can bounce the other one around. Even with all that, they only reduce rotational latency by a factor of two and it's not practical to "multiply" that factor by re- applying the technique. Digressing a bit: When you start predicting what's going to change and by how much, you have to keep power consumption in mind. Faster seek times are difficult, and have come very slowly, because there's a power issue involved. You're accelerating the head/arm assembly. If you want it to move faster, you have to make it lighter or push it harder. If you make it lighter, it has to become stronger at the same time, to withstand greater acceleration. If you push it harder, you have to be able to supply the power. (It's an interesting exercise to calculate the power required; it's straightforward F=ma and conversion-factors stuff once you know the mass you have to move.) You also have to be able to dissipate the heat from the energy used in seeking. The combination of smaller form factors and tighter cabinets means this is getting harder. All of this adds up to a statement that seek times are still only improving gradually; don't plan on radical improvements in the conventional style of disk drive. The usual approach to disk arrays doesn't help rotational latency: The data you want starts somewhere, so you have to wait for it to come around on whatever disk it's on. -- Dick Dunn rcd@ico.isc.com uucp: {ncar,nbires}!ico!rcd (303)449-2870 ...Lately it occurs to me what a long, strange trip it's been.