Disk storage

Access methods

Digital disk drives are block storage devices. Each disk is divided into logical blocks (collection of sectors). Blocks are addressed using their logical block addresses (LBA). Read from or write to disk happens at the granularity of blocks.

Originally the disk capacity was quite low and has been improved in one of several ways. Improvements in mechanical design and manufacture allowed smaller and more precise heads, meaning that more tracks could be stored on each of the disks. Advancements in data compression methods permitted more information to be stored in each of the individual sectors.

The drive stores data onto cylinders, heads, and sectors. The sector unit is the smallest size of data to be stored in a hard disk drive, and each file will have many sector units assigned to it. The smallest entity in a CD is called a frame, which consists of 33 bytes and contains six complete 16-bit stereo samples (two bytes × two channels × six samples = 24 bytes). The other nine bytes consist of eight CIRC error-correction bytes and one subcode byte used for control and display.

The information is sent from the computer processor to the BIOS into a chip controlling the data transfer. This is then sent out to the hard drive via a multi-wire connector. Once the data is received onto the circuit board of the drive, they are translated and compressed into a format that the individual drive can use to store onto the disk itself. The data is then passed to a chip on the circuit board that controls the access to the drive. The drive is divided into sectors of data stored onto one of the sides of one of the internal disks. An HDD with two disks internally will typically store data on all four surfaces.

The hardware on the drive tells the actuator arm where it is to go for the relevant track, and the compressed information is then sent down to the head, which changes the physical properties, optically or magnetically, for example, of each byte on the drive, thus storing the information. A file is not stored in a linear manner; rather, it is held in the best way for quickest retrieval.

Rotation speed and track layout

Mechanically, there are two different motions occurring inside the drive. One is the rotation of the disks inside the device. The other is the side-to-side motion of the head across the disk as it moves between tracks.

There are two types of disk rotation methods:

• constant linear velocity (used mainly in optical storage) varies the rotational speed of the optical disc depending upon the position of the head, and
• constant angular velocity (used in HDDs, standard FDDs, a few optical disc systems, and vinyl audio records) spins the media at one constant speed regardless of where the head is positioned.

Track positioning also follows two different methods across disk storage devices. Storage devices focused on holding computer data, e.g., HDDs, FDDs, and Iomega zip drives, use concentric tracks to store data. During a sequential read or write operation, after the drive accesses all the sectors in a track, it repositions the head(s) to the next track. This will cause a momentary delay in the flow of data between the device and the computer. In contrast, optical audio and video discs use a single spiral track that starts at the innermost point on the disc and flows continuously to the outer edge. When reading or writing data, there is no need to stop the flow of data to switch tracks. This is similar to vinyl records, except vinyl records started at the outer edge and spiraled in toward the center.

Interfaces

The disk drive interface is the mechanism/protocol of communication between the rest of the system and the disk drive itself. Storage devices intended for desktop and mobile computers typically use ATA (PATA) and SATA interfaces. Enterprise systems and high-end storage devices will typically use SCSI, SAS, and FC interfaces in addition to some use of SATA.

Basic terminology

Disk

Generally refers to magnetic media and devices.

Disc

Required by trademarks for certain optical media and devices.

Platter

An individual recording disk. A hard disk drive usually contains a set of platters though sometimes only one for low capacity models. Developments in optical technology have led to multiple recording layers on DVDs.

Spindle

The spinning axle on which the platters are mounted.

Rotation

Platters rotate; two techniques are common:

• Constant angular velocity (CAV) keeps the disk spinning at a fixed rate, measured in revolutions per minute (RPM). This means the heads cover more distance per unit of time on the outer tracks than on the inner tracks. This method is typical with computer hard drives.
• Constant linear velocity (CLV) keeps the distance covered by the heads per unit time fixed. Thus the disk has to slow down as the arm moves to the outer tracks. This method is typical for CD drives.

Track

The circle of recorded data on a single recording surface of a platter.

Sector

A segment of a track. 512 bytes traditionally in older hard drives(those made before 2010 and some enterprise drives under 4TB). Newer advanced format drives use 4KB sectors. Optical media uses 2KB sectors.

Low level formatting

Establishing the tracks and sectors.

Head

The device that reads and writes the information—magnetic or optical—on the disk surface.

Arm

The mechanical assembly that supports the head as it moves in and out.

Seek time

Time needed to move the head to a new position (specific track). Usually measured in milliseconds (ms) for hard drives.

Rotational latency

Average time, once the arm is on the right track, before a head is over a desired sector. This is how long it takes for one rotation to complete of a disk to reach a sector, for example 7200rpm drives have a rotational latency of 4.17 milliseconds.

Data transfer rate

The rate at which user data bits are transferred from or to the medium. Technically, this would more accurately be entitled the "gross" data transfer rate. Often measured in MB/s.