Intro to Computer Systems

Chapter 8: Mass Storage

Magnetic Media

Hard Disks

Hard disks are the most common form of mass storage in modern computers. They store data with the help of a special magnetic disk known as a platter; by modifing the magnetic field on different parts of the platter, it can store a series of binary digits.

Hard disks store binary data by the magnetic orientation of different areas on a disk platter.
Hard disks store binary data by the magnetic orientation of different areas on a disk platter.

Hard disks are known as mechanical storage: they work by spinning the magnetic platters, and have an arm that can move from one section of the disk to the other. In order to read and write data, a "head" is placed at the end of the arm mechanism.

The amount of data that can be recorded for a given unit area of space on the platter is called the aereal density, and increasing aereal density is one of the primary methods hard disk manufacturers have used to increase performance and capacity.

Modern consumer-level hard disks can store from 20 to 3000 gigabytes (GB) of data.

A hard disk with its air-tight cover removed. The platter and armature is clearly visible.
A hard disk with its air-tight cover removed. The platter and armature is clearly visible.

Because of the high speeds, moving parts, and close tolerances (the disk heads coast just a fraction of a millimetre above the platter surface!) they are susceptible to shock and rough handling.

Performance Considerations

The nature of hard disk construction (rotating platter/s of magnetic media, accessed through a read/write head) mean that there are some common performance characteristics to all magnetic hard disks, regardless of their age.

The access time (latency) of a hard disk is determined by its seek time and rotational latency.

Seek Time

A hard disk platter is arranged in a series of concentric circles, or tracks, which are then themselves arranged into sectors. Data is stored (and referenced by the file system) as residing in certain sector(s) of a particular track(s).

The seek time of a magnetic hard disk refers to the time taken for the heads to align themselves in the correct track, in order to read the sector(s) relevant to the operation it is performing.

An average seek time for a desktop hard disk is approximately 9 ms. Higher-end server hard disks will be significantly faster than this, owing to their construction for performance (over cost effectiveness).

Rotational Latency

The platters rotate at a constant speed, known as the spindle speed, measured in revolutions per minute (rpm). Typical desktop hard disks rotate at 7200rpm, whereas power-saving archival hard disks rotate between 5400 and 5900rpm. High speed desktop hard disks, and those used in servers, typically rotate at 10000rpm.

The faster the platters spin, the less time it takes for a given sector on the disk to fall under the read/write head for processing. Thus, rotational latency is in reverse proportion to the spindle speed of the platters.

There are, however, downsides to rotating the platters faster: they consume more power, and are louder.

Data Fragmentation

Files on a hard disk can be fragmented, similarly to how memory fragments from multiple allocations and deallocations. After a hard disk is used for a while, it is not uncommon for files to be split into portions, with each part stored in a different physical location on the platers.

Fragmentation manifests itself as slower performance; a given data file is no longer in a single location, and thus requires multiple seeks and accesses to gather all components.

Transfer Rates

The effective data transfer rate of a hard disk is bounded by two types of transfers:

The internal transfer rate is bounded by the limitations of the platters and heads, such as the speed at which data is read off the rotating platter, and any movements the heads need to do in order to sustain data transfer. Also, since the platters rotate at a constant speed (constant angular velocity), the outer sections of the disk have a higher transfer rate than the inner sections.

Modern consumer grade hard disks can perform this task at approximately 1 Gbit/sec.

The external transfer rate is typically much higher than the internal rate due to disk caches and high-speed mass storage interfaces like Serial ATA - suggesting a maximum of several gigabits a second (e.g. 3 Gbit/sec Serial ATA). Thus, "burst" transfer rates for hard disks tend to be higher than the magnetic platters would suggest.

Removable Magnetic Storage

Removable storage today is typically through the use of keychain-sized solid state disks with USB connectors, and occasionally recordable optical media - but before these were commonplace, removable forms of magnetic storage were king.

This could be traced back to the "punch cards" and huge magnetic tape reels of the mainframes of yesterday, however from a desktop computing perspective a more appropriate starting point is the floppy disk.

Floppy Disks

A floppy disk works on the same magnetic principle as a hard disk, however the data is far less densely packed. They are named "floppy disks" because behind the protective plastic cover, the magnetic recording medium is flexible.

The 3.5 inch floppy disk (with 1.4 MB capacity) was a common sight through the 1990s.
The 3.5 inch floppy disk (with 1.4 MB capacity) was a common sight through the 1990s.

There are many different floppy disk standards which dictated both their physical appearance and internal characteristics; the most common physical standards are the 5.25 inch and 3.5 inch form factors. Depending on the density of the recording system, these two sizes were commonly capable of storing up to 1.2 and 1.4 MB, respectively. (Future schemes allowed even higher capacities, but they never really caught on.)

Higher Density Media

Due to the floppy disks' low density and rotation speed, they were often annoyingly slow. When this started to become a significant issue in the early 90s, companies started to develop and market alternatives, each hopeful that their design would become the "next floppy disk".

A SyQuest removable cartridge drive.   A stack of SyQuest cartridges.
A SyQuest removable cartridge drive (left) and cartridge (right). The plastic disk cartridges housed a hard disk platter. Photo: SyQuest

SyQuest developed a cartridge-based system which in essence was a two-piece hard disk. The drive unit stored the motors and arm mechanisms, and the platters were made removable, kept protected by a plastic cartridge. These were popular amongst the graphics and publishing industries throughout the 80s and early 90s for their convenience (they were one of the very few industries that required frequent transportation of large data sets).

Iomega's ZIP drive system was the most popular high-volume removable storage system in the 1990s.
Iomega's ZIP drive system was the most popular high-volume removable storage system in the 1990s.

Iomega developed the ZIP disk system in 1994, which proved to become the most popular amongst the general public. The ZIP disk system allowed 100 MB (and later, 250 and 750 MB) to be stored onto a small cartridge about the same size as a 3.5 inch floppy disk. Unlike the SyQuest system, the drive and cartridges were closer to a floppy disk in design; this made them far cheaper.

In the end, none of these standards enjoyed "floppy disk" levels of success. Most of the floppy disk's perseverance in the marketplace came from the facts that just about every computer could accept them, and the disks were so cheap they became throw-away items.

Computer users gravitated towards one of the optical media formats for high-capacity data storage. This fit closer with the floppy disk model of success; just about every computer has a DVD drive of some description, and even these are starting to become somewhat obsolescent with the rise of solid-state media connected through USB.