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Unformatted text preview: , all heads are positioned on the same cylinder.
The disk surface spins at a fixed rotational rate The read/write head is attached to the end of the arm and flies over the disk surface on a thin cushion of air.
read/write heads spindle
arm By moving radially, the arm can position the read/write head over any track. spindle (a) Single-platter view Figure 6.10: Disk dynamics. (b) Multiple-platter view The read/write head at the end of the arm ﬂies (literally) on a thin cushion of air over the disk surface at a height of about 0.1 microns and a speed of about 80 km/h. This is analogous to placing the Sears Tower on its side and ﬂying it around the world at a height of 2.5 cm (1 inch) above the ground, with each orbit of the earth taking only 8 seconds! At these tolerances, a tiny piece of dust on the surface is a huge boulder. If the head were to strike one of these boulders, the head would cease ﬂying and crash into the surface (a so-called head crash). For this reason, disks are always sealed in airtight packages. Disks read and write data in sector-sized block...
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