Hard disk drive

 

Hard Disk Drive

A hard disk drive, also known as hard drive or HDD, is a secondary data storage device used to store and retrieve data or information. The primary characteristic of hard drives are the amount of data it can store called capacity, and performance. The capacity is measured in gigabytes or terabytes. Hard drives performance can refer to read speeds, write speeds, latency and the seek time.

Internal Structure of Hard Disk Drive

The Platter: The media is the hard metallic disk made of Aluminum and coated with iron oxide which gives a typical rust brown look. Unlike the floppy disk drive, the media in the hard disk drive is permanently fixed to the drive mechanism; hence it is also called fixed disk drive. Depending on the capacity of storing data there could be more than one platter (disk). Usually 2, 3, 4, etc. since both the sides of the disk platter are coated with the magnetic material, it provides additional storage space.

Read/Write Heads: The heads are an interface between the magnetic media where the data is stored and electronic components in the hard disk. The heads convert the information, which is in the form of bits to magnetic pulses when it is to be stored on the platter and reverses the process while reading.

Each platter has two read/write heads, one mounted on the top and the other one at the bottom. These heads are mounted on head sliders, which are suspended at the ends of heads arms. The head arms are all fused into a singular structure called actuator, which is responsible for their movement.

Voice coil actuator: Voice coil method of actuation is done usually in large capacity drives with high reliability. The voice coil mechanism moves he head carriage assembly by pure electro-magnetic force. The construction of this is analogous to an audio speaker which uses a stationary magnet surrounded construction of this is analogous to an audio speaker which uses a stationary magnet surrounded by a voice-coil (hence the name ) that is connected to the paper cone.

Hard Disk Spindle Motor: The spindle motor, also sometimes called the spindle shaft, is responsible for turning the hard disk platter, allowing the hard drive to operate. It provide stable, reliable and consistent turning power for thousands of hours of continuous use, to allow the hard disk to function properly. Many drive failures are actually failures with the spindle motor, not the data storage systems.

Hard Disk Geometry

The Hard Disk Geometry refer to the five special values: the heads, cylinders, sector per track, write precomp and landing zone.

Heads: The number of heads for a specific hard drive describes, rather logically, the number of read/write heads used by the drive to store data. Every platter requires two heads. For e.g. if a hard drive has four platters, than it needs to have eight heads.

Cylinder: A platter has two sides with concentric tracks. Two track that are align exactly with each other (one on the bottom of the platter, and one on the top) from a cylinder. If there are two or more platters, all pairs of tracks that line up with each other make up the cylinder. If there are three platters, for e.g. each cylinder has six tracks (three pairs).

Write Precompensation Cylinder: Older hard drives had a real problem with the fact that sectors towards the inside of the drives were much smaller than sectors toward the outside. To handle this, an older drive would write data a little further apart once it got to a particular cylinder. This cylinder was called the Write precompensation (write precomp) cylinder, and the PC had to know which cylinder began this wider spacing. Hard drives no longer have this problem, making the write precomp setting obsolete.

  

Landing Zone: On older hard drives with stepper motors, the landing zone value designated an unused cylinder as a “parking place” for the read/write heads. Old stepper motor hard drives needed to have the read/write heads parked before being moved in order to avoid accidental damage. Today’s voice coil drives park themselves whenever they’re not accessing data, automatically placing the read/write heads on the landing zone. As a result, the BIOS no longer needs the landing zone geometry.

Sector per Track: Imagine cutting the hard drive like a birthday cake, cutting all the tracks into tens of thousand of small slice. Each slice is called a sector, and each sector stores 512 bytes of data.

Interleaving: Interleaving is the process through which gaps are placed between two sectors on the platter of a disk. This was done in earlier days when the computers were not quick enough to read continuous stream of data. Without interleaving there would be no gaps between the sectors and data would arrive immediately before the reading unit is ready. Due to this to read the same data a complete rotation of the disk would be required again.

Hard Disk Interface

Hard disk interface re those by which a hard disk is connected to the PC and act as a communication channel over which all the data flows that is read or written to the hard disk.

Hard disk drives are accessed over one of a number of bus types, including parallel ATA (PATA) and also called IDE or EIDE, Serial ATA (SATA), SCSI.

Enhanced Small Device Interface (ESDI): The first attempt at improving the original ST-506/ST-412 hard disk interface was the Enhanced Small Device Interface or ESDI. It had a maximum theoretical bandwidth of 24Mbits/second (fairly fast for those days). Though in practice the limit was about half of that. In the late 1980s ESDI suffered under competition from IDE/ATA in the mainstream market and from SCSI in the high-end market, both of which offered significant advantage over ESDI, such as simpler configuration, lower cost and improved performance.

Integrated Drive Electronics / AT attachment (IDE/ATA) Interface: Parallel ATA, originally IDE and the standardized under the name AT Attachment (ATA). The 40-pin IDE/ATA connection transfer 16 bits of data at a time on the data cable. The data cable was originally 40-conductor, but later higher speed requirements for data transfer for the hard drive led to an “Ultra DMA mode; known as UDMA”. The interface for 80-conductor only has 39 pins, the missing pin acting as a key to prevent incorrect insertion of the connector to an incompatible socket, a common cause of disk and controller damage. IDE(PATA) transfer data from the speed of 5MB/sec up to 133MB/sec.

SCSI (Small Computer System Interface): This interface was introduce as a method of connecting multiple peripherals to computers. Based on a parallel bus structure, with each device having a unique ID (or address), the SCSI bus will support up to seven devices plus the host adapter. Newer “wide” interface, used almost exclusively for hard drives, can support up to 15 devices plus the host controller, and can transfer data at burst speed of up to 320 MB/sec.

Because of the multiple device support and extended cable length (up to 6 meters for SCSI-2), the higher transfer rate, and the ability to install multiple host adapters on the motherboard on in available connectors, the SCSI interface is used most often to connect external devices such as scanner, CD-ROMs, CD duplicator and multi-drive storage enclosures, while at the same time connecting to SCSI devices internally, usually on the same adapter.

SATA Interface: SATA stands for Serial Advanced Technology Attachment. It is a new high-speed serial interface for mass storage that will eventually replace Parallel ATA (PATA), the current mass storage attachment standard. The SATA standard defines a data cable with seven conductors, three grounds and four active data lines in two pins.

It has the advantage of increased bandwidth 150-300 MB/s depending upon the standard used as compared to 100 MB/s for PATA, thinner, longer cables, lower voltages and no jumpers.