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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Geometry and Low-Level Data Structures | Hard Disk Data Tracks, Cylinders and Sectors ]

Cylinder and Head Skew

Sector interleaving was once used on older hard disks to ensure that the sectors were efficiently spaced on the track. This was needed to ensure that sector #2 didn't rotate past the head while sector #1 was being processed. The high-speed disk controllers on modern drives are now fast enough that they no longer are a performance-limiting factor in how the sectors on the disk are arranged. However, there are other delay issues within the drive that require spacing to be optimized in even the fastest drives, to maximize performance. And unlike the interleaving situation, these delays are caused by electromechanical concerns and are therefore likely to be with us for as long as hard drives use their current general design.

The first issue is the delay in time incurred when switching between cylinders on the hard disk, called appropriately enough, cylinder switch time. Let's imagine that we "lined up" all of the tracks on a platter so that the first sector on each track started at the same position on the disk. Now let's say that we want to read the entire contents of two consecutive tracks, a fairly common thing to need to do. We read all the sectors of track #1 (in sequence, since we can use a 1:1 interleave) and then switch to track #2 to start reading it at its first sector.

The problem here is that it takes time to physically move the heads (or more actually, the actuator assembly) to track #2. In fact, it often takes a millisecond or more. Let's consider a modern 10,000 RPM drive. The IBM Ultrastar 72ZX has a specification of only 0.6 milliseconds for seeking from one track to an adjacent one. That's actually quite fast by today's standards. But consider that in that amount of time, a 10,000 RPM drive will perform approximately 10% of a complete revolution of the platters! If sector #1 on track #2 is lined up with sector #1 on track #1, it will be long gone by the time we switch from track #1 to track #2. We'd have to wait for the remaining 90% of a revolution of the platters to do the next read, a big performance penalty. This problem isn't as bad as the interleave one was, because it occurs only when changing tracks, and not every sector. But it's still bad, and it's avoidable.

The issue is avoided by offsetting the start sector of adjacent tracks to minimize the likely wait time (rotational latency) when switching tracks. This is called cylinder skew. Let's say that in the particular zone where tracks #1 and #2 are, there are 450 sectors per track. If 10% of the disk spins by on a track-to-track seek, 45 sectors go past. Allowing some room for error and controller overhead, perhaps the design engineers would shift each track so that sector #1 of track #2 was adjacent to sector #51 of track #1. Similarly, sector #1 of track #3 would be adjacent to sector #51 of track #2 (and hence, adjacent to sector #101 of track #1). And so on. By doing this, we can read multiple adjacent tracks virtually seamlessly, and with no performance hit due to unnecessary platter rotations.

The same problem, only to a lesser degree, occurs when we change heads within a cylinder. Here there is no physical movement, but it still takes time for the switch to be made from reading one head to reading another, so it makes sense to offset the start sector of tracks within the same cylinder so that after reading from the first head/track in the cylinder, we can switch to the next one without losing our "pace". This is called head skew. Since switching heads takes much less time than switching cylinders, head skew usually means a smaller number of sectors being offset than cylinder skew does.

These two diagrams illustrate the concept of cylinder and head skew. Assume that these platters spin
counter-clockwise (as seen from your vantage point) and that they are adjacent to each other (they might
be the two surfaces of the same platter.)  They each have a cylinder skew of three, meaning that adjacent
tracks are offset by three sectors. In addition, the platter on the right has a head skew of one relative
to the one on the left. (Of course, real drives have thousands of tracks with hundreds of sectors each.)

Both cylinder and head skew must be simultaneously "overlaid" onto all the tracks of the hard disk, resulting in a "two-dimensional pattern" of sorts, with different offsets being applied depending on the specific timing characteristics of the disk. The layout of the tracks is adjusted to account for cylinder skew and head skew, based on the way the designers intend the hard disk to store sequential data. All of the details are taken care of by the controller. This is one reason why having integrated, dedicated drive electronics on the disk itself, is such a good idea. No universal, external controller could possibly know how to take all these hard disk characteristics and performance requirements into account.

Next: Sector Format and Structure

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