Magnetic Hard Disk Drives

Magnetic Hard Disk Drives

Commonly known as hard drives, the hard disk drive (HDD) permanently store data and provide quick access to it. Except for external and removable hard drives, typical hard drives are placed inside the computer (semi-permanently mounted) with no external access. A hard drive can hold much more information as compared to other storage devices. The hard drive or conventional drives use a magnetic storage medium.

Hard disk drive systems comprise three important components:

Controller: The control of the drive lies with this component. The operation of the drive and encoding of data onto the platters — all this is controlled by the controller chip. The controller chip controls how the signal is to be sent by the data to the different motors of the drive, and also controls receipt of signal by the data from the sensors in the drive. Modern day hard disk drive technologies include the controller and drive as a single unit, for example, SATA.

Hard Disk: Being the physical storage medium for hard disk drive systems, hard disk comprises small disks (under 1 to 5 inches in diameter) storing information. These are also called as platters and these small discs are stacked together in an enclosure.

Host Bus Adapter (NBA): It converts signals received from the controller into a form that can be interpreted by the computer. Modern day motherboards have the host adapter into their circuitry. This arrangement offers headers for drive-cable connections. Some modern adapters and legacy host adapters incorporate the hard drive controller circuitry.

To avoid any contamination in the hermetically sealed drive casing, a hard drive is constructed in a clean room. After sealing the casing, a sticker typically seals one or more screws. The precision components can be damaged by even the smallest of the contaminants. The hard drive’s external shell guards against any such contaminating intrusion.

There are one or more platters within the hard drive’s sealed case. The read/ write heads store the actual data. A mechanism with the mounted heads moves these heads in sync across both surfaces of all the platters. In older drives, a steeper motor positions the heads at discreet points along the surface of the platters. These platters spin at thousands of revolutions per minute on a spindle that is mounted on a hub. For a more analogue movement, newer drives use voice coils. This arrangement results in reduced data loss as the circuitry senses the data location by a servo scheme. Even in cases where changes in the physical disc geometry have shifted the data, the circuitry can still sense it.

The inherent flaws of the platters are mapped by the newer drives’ manufacturing technologies. This mapping enables the drive controllers to avoid placing data in these risky locations. In drive preparation, this phase additionally creates concentric rings or tracks. These tracks are magnetically drawn around the platters’ surface. The magnetic domains representing the smallest unit of storage on the platters are known as sectors. Commonly, each magnetic drive sector stores only half KB (512 bytes) of data.

The hard drive’s controller knows about the exact placement pattern of sectors within the disk assembly. The BIOS directs the controller as the controller writes information to the drive and reads the drive’s geometry. However, the BIOS does not necessarily have understanding about the drive’s geometry. For instance, the BIOS supports only upto 63 sectors on one track. In numerous hard drives, there are much more than 63 sectors per track. Thus, a translation is required from where the BIOS interpret the information to be written to where the controller has actually written the information. The controller enables the BIOS to detect the drive’s geometry by reporting understandable dimensions to the BIOS.

The basic hard drive geometry comprises three components: number of read/write heads, number of cylinders in the disk assembly and number of sectors on each disk. These set of values are called CHS. CHS stands for cylinders/heads/ sectors. The number of tracks placed on a single surface of a single platter depicts the number of cylinders. When vertically connected, the collection of similar number of tracks on writable surfaces of the hard drive assembly looks similar to a geometric cylinder (thus named as cylinders).

HDD has various speeds as discussed below:

HDD Speeds Faster electronics within the HBA and controller demand data at higher and higher rates. But, the information can be accessed as quickly as a fixed rate if the platters spin at a constant rate. To enable quick access of the information to the electronics, manufacturers increase the spin rate of platters with every new generation of drive. This leads to unpredictable period of coexistence of multiple speeds in the marketplace, at least until there is fall in the demand for one or more speeds. In conventional magnetic hard disk drives, the listed spin rates for the platters have been used:

  • 5400 rpm
  • 7200 rpm
  • 10,000 rpm
  • 12,000 rpm

A higher revolution per minute (rpm) ratings means quicker movement of data, but these increased disk access speeds are not beneficial to many applications. To choose faster drives means more expense per byte of capacity when you use an application for this kind of performance. For example, housing the partition of operating system or highly disk-intensive Programs. Laptops require lower speeds as higher-speed drives can lead to higher heat production and battery usage.

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