Hard Disk Drive

 Let’s look a bit further into the dynamics of the hard disk drive. Disk drives are a marvel of engi-neering technology. For years, electronic industry analysts were predicting the demise of the hard drive. Economical semiconductor memories that were as cost effective as a hard disk were always “a few years away.” However, the disk drive manufacturers ignored the pundits and just continued to increase the capacity and performance, improve the reliability and reduce the cost of the disk drives. Today, an average disk drive costs about 60 cents per gigabyte of storage capacity. Consider a modern, high-performance disk drive. Specifically, let’s look at the Model ST3146807LC from Seagate Technology®4. Following are the relevant specifications for this drive:Rotational Speed: 10,000 rpm•Interface: Ultra320 SCSI•Discs/Heads: 4/8•Formatted Capacity (512 bytes/sector) Gbytes: 146.8•Cylinders: 49,855•Sectors per drive: 286,749,488•External transfer rate: 320 Mbytes per second•Track-to-track Seek Read/Write (msec): 0.35/0.55•Average Seek Read/Write (msec): 4.7/5.3•Average Latency (msec): 2.99
What does all this mean? Consider Figure 1.5. Here we see a highly simplified schematic diagram of the Seagate Cheetah®disk drive discussed above. The hard disk is comprised of 4 aluminum platters. Each side of the platter is coated with magnetic material that records the stored data. Above each platter is a tiny magnetic pick-up, or head, that floats above the disk platter (disc) on a cushion of air. In a very real sense, the head is flying over the surface of the platter, a distance much less than the thickness of a human hair. Thus, when you get a disk crash, the results are quite analogous to when an airplane crashes. In either case, the airplane or the read/write head loses lift and hits the ground or the surface of the disk. When that happens, the magnetic material is scrapped away and the disk is no longer usable

Each surface contains 49855 concentric tracks. Each track is discreet. It is not connected to the adjacent track, as a spiral. Thus, in order to move from one track to an adjacent track, the head must physically move a slight amount. All heads are connected to a common shaft that can quickly and accurately rotate the heads to a new position. Since the tracks on all the surfaces are in vertical alignment, we call this a cylinder. Thus, cylinder 0 contains 8 tracks from the 4 platters. Now, how do we get a 146.8 Gbyte drive? First, each platter contains two surfaces and we have four platters, so that the total capacity of the drive will be 8 times the capacity of one surface. Each
sector holds 512 bytes of data. By dividing the total number of sectors by 8, we see that there are 35,843,686 sectors per surface. Dividing again by 49855, we see that there are approximately 719 sectors per track. It is interesting that the actual number is 718.96 sectors per track. Why isn’t this value a whole number? In other words, how can we have a fractional number of sectors per track? There are a number of possibilities and we need not dwell on it. However, one possibility is that the number of sectors per track is not uniform across the disk surface because the sector spacing changes as we move from the center of the disk to the outside. In a CD-ROM or DVD drive this is corrected by changing the rotational speed of the drive as the laser moves in and out. But, since the hard disk rotates at a fixed rate, changing the recording density as we move from inner to outer tracks makes the most sense. Anyway, back to our calculation. If each track holds 719 sectors and each sector is 512 bytes, then each track holds 368,128 bytes of data. Since there are 8 tracks per cylinder, each cylinder holds 2,945,024 bytes. Now, here’s where the big numbers come in. Since we have 49855 cylin-ders, our total capacity is 146,824,171,520 bytes, or 146.8 Gbytes. Before we leave the subject of disk drives, let’s consider one more issue. Considering the hard drive specifications, above, we see that access times are measured in units of milliseconds (msec), or thousandths of a second. Thus, if your data sectors are spread out over the disk, then accessing each block of 512 bytes can easily take seconds of time. Comparing this to the time required to access data stored in main memory, it is easy to see why the hard drive is 10,000 times slower than main memory