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Servo-Sectors and Performance

We first encountered this parameter when investigating the WD2500JB model from Western Digital (see our review called Western Digital WD2500JB HDD: More than Drivezilla?!). We couldn’t understand then why the new drive had much better parameters compared to its predecessor, as they but slightly differed in the linear speed and other characteristics. We found an explanation, though, with the help of Mikhail Mavritsyn. The WD2500 just had considerably more servo identifiers or SIDs. To clear out what these identifiers are, we should delve deeper into the principles of operation of current hard disk drives.

Quite a long time ago (for the specific dates refer to our article called The Last IBM Drive: Deskstar 180GXP HDD Details), the growth of areal density had led to the abandonment of a separate surface for storing “navigational” data needed by the heads positioning mechanism. They created the so-called embedded servo instead. The problem read as follows: the width of the data tracks was so small that it became impossible to seek for and stay on a track using the remote servo control. Even the thermal expansion of materials could make it impossible to find a track where expected, but there were also such things as assembly inaccuracy and mechanism wear. That’s why they developed a control system based on servo information built directly into the data storage area. The tracks were divided into groups of sectors, with servo identifiers in between. The magnetic head chosen at the moment is always reading and trying to identify servo information among data proper. Finding it, the head learns its present bearings and calculates the way to the necessary data, relative to its current position. If the data are not on the same track with the head, a precisely dosed impulse moves the head closer to the required track. If the data are on the same track, it’s only necessary to wait a little for the platters to rotate by the necessary degree.

It is here that a strong correlation between the number of SIDs and the HDD’s speed is observed. The higher the track density is, the more tracks pass by the heads per a time unit during the positioning operations. Since the speed of the electronics isn’t infinite, and the accuracy of the mechanics isn’t absolute, the drive doesn’t always hit precisely on the necessary track. The electronics of course doesn’t know where the heads have landed until a strip of servo data have passed beneath them, and thus cannot start to read/write data or to correct the position of the heads further. In other words, the growth of the track density should result in a proportional increase of the number of SIDs, if we don’t want the product to perform worse. Let’s see how this theoretical premise correlates with the practice. The information on the amount of servo data per track for several models of hard disk drives is again courtesy of Mikhail Mavritsyn (by the way, Maxtor sometimes puts this number in the documentation).

Let’s start with the above-mentioned case study of discs from Western Digital.

The total density growth of 25 percent was mainly achieved through the increase of the number of tracks rather than of the number of sectors per track. But the increase of the number of SIDs covered the track density growth with interest, and we enjoyed a clearly higher performance of the new model. But what about the currently tested product, the Deskstar 7K250? Let’s examine its closest ancestors:

 
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