Why do Hard Drives and other storage media record from the core out?

Factual Questions
1

Hard drives are round flat disks. They record starting at the middle, the core. The FAT Tables on a Windows machine are stored here. I suppose that in the old days, the armature that read the hard drive disks was “at rest” laying in the middle and so it might have made more sense mechanically to have the most important and most-often read files at the core. ( my WAG, if this is off-base, please correct me ).

However, nowadays with burnable CD/DVD type of media, the record and read mechanism is a laser mounted into a track. It’s irrelevant where the laser is on the track, it can read and write and slide back and forth rapidly with ease. Why are CD’s and DVD’s also written from the core out to the edge? Nostalgia? Some other real reason?

Cartooniverse

2

It’s probably mainly nostalgia, but it also makes it possible to have ‘miniature’ CDs.

3

I’d say this is mainly because as you get closer to the center, since the rotational speed of the disc is constant your data read/write speed increases. Having the disc start at the center allows an almost empty disc have all of its data in the “fast” area.

4

It’s actually the other way around, the outer edges allow a higher data transfer speed because more area is swept per revolution.

5

To expand on what was said above.

Let’s imagine a disk where the innermost track has a 1" dia., and the outermost track is 5" dia. The other ring has 5x the magnetic material, and could potentially record 5x the data. Therefore, if the cost/complexity of the control electronics permit, we should ideally put 5x the number of constant-size sectors in the outermost ring.

Reading multiple sectors on the same track is faster than switching tracks, because the disk is spinning at a constant high speed–7200-10,000 RPM fpr today’s inexpensive IDE drives, or roughly 6-8.3 msec to read the entire track (ignoring logical skewing, which I’m not even sure today’s drives even do anymore) However, switching tracks requires moving the comparatively large physical arm, and then aligning it exactly with the new track through feedback. It just so happens that the time to switch to an adjacent track [track seek time] is typically comparable to the time required to read all the sectors on one track. Lord help you if you have to jump 10 tracks becuase your file is fragmented.

As a result, the inside tracks are “slower” (less data per revolution, more switching to other tracks). Since every [standard] computer has at least one boot drive, it makes sense to put the boot/OS information (which you only read once) in these slower sectors, and use faster [more data per revolution] outer sectors for normal operations. Which would you prefer: a computer that took an extra 100 msec to boot, or a computer that might take an extra 100 msec when you’re in the middle of using it?

The difference can be substantial. Reading one Outermost track (again ignoring logical skewing) on a modern drive takes less than half the time of reading several innermost track to get the same amount of data (MUCH less than half, actually, because I’ve left out other major delays in track-to-track reading, like re-synching yourself with the new track, so you know which sector you’re reading – after all, the platter is still spinning as you’re moving the arm) We don’t notice the difference directly, because a) it still only adds up to a fraction of a second; b) drives are really slow compared to the rest of the computer, so any drive access is like molasses; and c) our operating systems, IDE, and applications often make extensive use of caching (“semi-autonomously reading ahead just in case we need the data”) rather than “reading on specific demand only”. In the aggregate, though, it’s quite noticeable, which is why defragmenting a disk (or reinstalling a key application) can cause a speed-up even if “nothing is wrong”.

There used to be “disk optimizing” utilities that let you choose where to put certain files (inner, middle or outer) but I haven’t seen those in years. They may not be considered “worth the time/effort” with today’s much faster, much bigger drives – or perhaps few users would make better choices than the few basic rules embedded in the OS.

6

Certainly less than half, even ignoring track switching. In your example, the outer edge would be at least five times faster.

7

There is a practical reason. At start up the disc has to come up to speed and an air film is formed between the surface of the disc and the pickup heads.
The velocity would be the lowest near the center of rotation thus mimimizing wear.

8

Don’t lump hard drives and CD/DVD drives in the same basket. Unless things have changed recently, all hard drives spin at a constant RPM regardless of where the head is (constant angular velocity). CD drives are spun more slowly when the head is reading the outer tracks (constant linear velocity). HDs are divided into sectors, more of which can be on the outer tracks (tracks are often grouped into zones with the same number of sectors per track within each zone). CDs read in fixed size data blocks, not sectors. HDs have concentric tracks; CDs have a spiral track.

So why do both start at the middle? To paraphrase the old punch line, everybody’s gotta start someplace. :slight_smile:

9

My WAG is that it didn’t matter when CDs were first invented, since they were were pressed and designed to be played back at a constant 1X. Then when recorders came along they had to follow the same scheme to be compatible with the players. Probably the same for DVD, since they can also play/record CDs.

10

Just to confuse things further it should be noted that CD drives can either be Constant Linear Velocity (CLV) or Constant Angular Velocity (CAV).

In CLV drives the spindle adjusts its RPMs on the fly to maintain the same speed of data passing the read head. So, the farther out the head goes the slower the drive spins.

In CAV drives the disk maintains a constant speed at all times.

So, a CLV drive is no faster reading data at the edge of the disk than it is at the center. A CAV disk can show a marked difference reading data in the center versus the edges (possibly as much as 50% slower or more at the center versus the edges).

Generally the faster CD drives (above 12x or so) are CAV drives. Which is better is a little hard to say. The CLV drive will give more consistent performance. A CAV drive on a 1/4 full disk might be slower than a similarly rated CLV drive. On a full disk the CAV drive will tear ahead of a CLV drive. Of course, there is a lag involved with a CLV drive adjusting RPMs so it is a give or take.

Personally I prefer CAV drives as they seem to run more smoothly by not having to adjust themselves as the read head moves about but I am not sure an actual “better” can be applied to either style as they both have advantages and disadvantages.

11

This is what I get for asking for The Straight Dope. :slight_smile: --pops two Tylenols, tries desperately to keep up here–

Am I reasonably correct in assuming that while you might sense a speed difference when CD ROM drives were 1x, at this point the “choke points” in a highspeed or even moderate-speed machine are SO fast, that you’d have trouble telling which kind of CD ROM reader was faster? A 45x, or higher, with 3 gigahertz, 1 Gig RAM, etc… how can one tell- and more to the point, in what case might it even matter?

12

Is there any reason it would make more sense to start at the outside? Perhaps it doesn’t matter either way, and it just strikes some as odd because we’re used to thinking about vinyl.

13

Many operating systems put the file housekeeping information (file allocation table or equivalents) half-way between the innermost and outermost tracks on the disk. This reduces the worst-case time for opening a file and starting to read data from the file.

14

For vinyl records it makes sense to have a design that enables the record to stop automatically when finished. This is done by having them stop at a common inside diameter, but the price we pay for this is having to select the start diameter manually to account for 7", 10", 12" and picture discs.

For optical discs (CD, DVD, Blu-Ray) we can detect the end of the disc by the cessation of data being read back, and the optical pickup unit (OPU) can be controlled by the system microprocessor. So it makes sense to have all optical discs start at a common inside diameter, and then move outwards.

The inside area of the disc also contains special data areas for the disc manufacturer’s recommended writing parameters, and the optimum power calibration (OPC) procedure is also performed in a special reserved area on the inside of the disc, just before the normal written data area.

The laser powers and patterns needed to get the best write quality will vary according to disc speed, and so for CAV writing we will need to update this parameters as we move across the disc. Usually this involves writing a series of zones, so if you read back the write quality of a written disc (jitter, errors etc) you may notice discontinuities between zone boundaries. Sometimes CAV writing is lots of little CLV zones, which is cheating a bit.

Someone had to set these standards when CDs were invented (a joint effort between Philips and Sony), and quite often the reasons are delightfully arbitrary. The size of the hole in a CD was made the same size as a Dutch 10-cent coin (15mm), and the 12cm diameter of a CD was chosen to be a handy size, but also to be able to fit the Sony CEO’s favourite rendition of Beethoven’s 9th. A 12cm disc gives about 74 minutes of music at the original CD spec, which fitted nicely.
The start diameter of an optical disc allows a bit of room for the clamper mechanism.