Why don't hard drives spin at other RPM's?

[corfe83]

I'm just curious, why hard drives always spin at certain speeds. For example, desktops are almost always 5.4k, 7.2, 10k, or rarely 15k.

My question is, why is it like this? Why isn't there such thing as an "8k rpm" or "7.5k rpm" drive? Are these set RPM speeds a certain sweet spot in terms of reliability and speed?

Do the motor manufacturers only make motors at these speeds?

Optical drives are always upping spin speeds - why can't hard drives (besides in set increments)?

Seriously, there's got to be some kind of reason. Does anyone know what it is? "That's how the industry does it" doesn't count as an answer

 

[newmachineoverlord]

If you're making a million motors for a million drives, you get better economies of scale if you limit the number of speeds to one or two. Plus each speed drive has to be reliability tested separately, speed tested separately, etc. so the more variety of speeds you have, the more it increases your overhead.

Reliability is a bigger issue with HD's than optical discs partly due to greater usage (everyone uses a HD pagefile, not everybody uses their cdrom daily) but also because excess heat can cause damage to the storage media. For excess heat to damage the storage media in an optical drive, you'd have to be using the same disc all the time, but even then it won't get as hot because HD's have multiple platters and therefore more friction. Thus reliability testing is a more significant expense with hard disks at a given rpm than with optical drives.

Performance measures differ between optical and magnetic drives. Hard disks measure performance in data transfer rates, MB/s, whereas optical drives tend to advertise spin speed as the sole performance measure. When it comes to buying a HD people are far more likely to actually look up a performance review. Thus, the engineers are motivated to spend time improving actual performance, rather than just increasing the number in front of the X. With optical drives, you need a big number in front of the X to move the product, and it needs to spin quietly to keep from getting returned.

 

[bob4432]

i have read a few articles about the actual mechanics of a hdd and i think that the speeds need to be constant due to everything that is going on mechanically inside. after reading the articles i am amazed that hdds even work, let alone as reliable as they are

i too would like to know why they pick the speeds they do, maybe i missed that in the articles.....

 

[Snooper]

Economics. Look at all the hardware that has to work correctly for a drive to work. When you do change the drive speed, you have reengineer ALL those other parts (drive controller, interfaces, mechanics, read head positioning hardware/firmware, etc.) to get it all working.

Then you have to test is all out and prove that it will actually work (much harder than it sounds). Plus, the HD manufacturers do not make all the parts for their drives. They out source a lot of parts.

So, when you are going to make a change, it better be a big enough change (say from 5400 to 7200 or from 7200 to 10k) that the cost involved can be paid for with higher prices on the drive. And now, all your competitors can buy these same parts to be installed in THEIR new xxxx rpm drive, so everyone is making the same speed drives. Again.

 

[Pariah]

The other people answered the main question, so I won't rehash it. There is nothing significant about the RPM's that are used. 10k and 15k are nice round numbers which is probably about the only reason they were chosen, keeping in mind that drives don't spin at exactly 10k RPM's or whatever their speed designation is. The other RPM's were probably chosen because they introduced a large enough performance increase from the generation they were replacing.

Optical drives up the motor speed because the discs they read have a constant set of specs. So the only way performance can be increased is by spinning the disc faster, if we ignore the ill-fated True-X drives. It's much easier to increase the speed of the motor when you know the drive will only be reading a disc with one format. With hard drives, the layout of the data on the platter is constantly changing, so to simplify the task of making faster drives, the motor RPM's are kept constant, along with the other reasons listed by other people above.

 

[13black]

In a hard drive the disk platter has to spin fast enough to keep the heads on a cushon of air. If you slow it down you get a head crash, not a good thing. You don't have the same limitation in an optical drive. Buy changing the speed of the disk you can store more data on the CD. I beleive data is writen to a CD from the center out. so if you reduce the speed of the disk as you move towards the outside you can write more data to the disk.

 

[Matthias99]

Originally posted by: 13black
In a hard drive the disk platter has to spin fast enough to keep the heads on a cushon of air. If you slow it down you get a head crash, not a good thing. You don't have the same limitation in an optical drive. Buy changing the speed of the disk you can store more data on the CD. I beleive data is writen to a CD from the center out. so if you reduce the speed of the disk as you move towards the outside you can write more data to the disk.

Rotational speed has no (direct) relationship to storage density.

It's just that a slower-moving disk (whether magnetic or optical) is 'easier' to write at a given density -- ie, you need more expensive/precise motors and read/write heads to work at a higher speed. There are also physical limitations -- you can't really spin CDs/DVDs at more than ~52x speed, because the disks just physically can't take it (yes, they sometimes break up at 52x speed if they're flawed; Mythbusters even did it in an episode). And they use some pretty exotic materials (hardened aluminum, glass, ceramics) in 10K/15K RPM hard drive platters. Stepping up to higher speeds than 15KRPM -- unless the platters are made smaller -- will require VERY strong platter materials.

CD/DVD drives these days are almost all CLV (Constant Linear Velocity) -- the drive spins faster (in degrees/second) when reading/writing near the middle, and slower near the outside, such that the speed of the media under the read/write head is somewhat constant. These days most drives are 'zoned', so it is still somewhat faster (in linear speed) near the outside, but only by about a factor of two.

http://en.wikipedia.org/wiki/Constant_linear_velocity