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The reason for the discrepancy between the just mentioned "internal performance" and the benchmark results is twofold. First, HDDs are using a 10bit / Byte conversion, therefore, the highest possible transfer rates at the OD would peak in the 64 MB/sec range. Second, and more importantly, the platters of modern drives do not only contain data, rather the data tracks are interrupted by so-called "Servo Bursts" or "Servo Wedges", depending on whether the physical mapping is used as basis for the nomenclature or else the actual data signaling sequence.

Servo wedges containing positional data are interleaved with the data. The head reads the positional data which are fed into a closed-loop feedback servo controller for precision-positioning of the head over the individual tracks.

Servo Bursts

Briefly, a "Servo Burst" is a wedge shaped sector of the platter that contains protected positional data that are being read by the head and fed into a close-loop feedback circuitry to the servo controller for adjusting the head's positioning on the target track. What this means is that the data areas on each track are interrupted by wedge shaped areas that hold the positional data. Those Servo Bursts are factory -installed / protected and cannot be erased / modified by the user. Early HDDs did not feature Servo Bursts, instead, amongst the multiple platters, one platter was dedicated to holding only positional data. Since all heads are moving in unison, this solution did suffice as long as the TPI (tracks per inch) density was relatively low, meaning that head positioning did not require ultimate precision.

A reference platter is used instead of servo wedges. The advantage is that there are no interruptions between LBAs, on the other hand, the increased TPI (tracks per inch) density used in more modern drives requires constant active repositioning and recalibration of the servo feedback loop for correct positioning of the head over the track (or cylinder).

LBAs can span across servo bursts, which will result in a performance hit on that particular LBA, however, since single LBA transfers carry a huge command overhead with them, the performance hit in this case is not important in the overall picture of the drive's performance.

Finally, there are some unused areas that are meant to accommodate the track to track repositioning of the head, along with a rotational offset of consecutive tracks in order to prevent excessive rotational latencies upon track switching. A lot of these data and alignment issues are what is considered secret sauce of each manufacturer.

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