Intro to Computer Systems

Chapter 8: Mass Storage

Optical Media

Optical Storage

Since the invention of the modern Compact Disc in 1982, the various technology off-shoots from this starting point have proven to become the most popular method of distributing large amounts of data.

Compact discs represent binary data by a pattern of pits in the disc surface. These are read by a laser and converted to computer data.
Compact discs represent binary data by a pattern of pits in the disc surface. These are read by a laser and converted to computer data.

The compact disc stores information by coding binary digits as microscopic "pits" (which are in effect, holes) in the disc surface. The pits (or lack thereof) can be read by a laser beam, and the binary digits decoded.

The original Compact Disc standard was only intended to store high-quality music. Through a process of constant refinement and development, there have been many extensions to the format to allow rewritability and increased capacity.

Laser Wavelengths

The way in which the storage capacities of optical media can be increased, is by using a shorter wavelength of laser light to read and record the surface. A shorter wavelength allows the data to be coded onto the disc in a more compact fashion.

Laser light appears to be of a certain colour, if it is part of the visible spectrum. Compact Discs use a near-infrared wavelength, and as such are invisible to the eye. DVDs use a wavelength that appears red to the eye. Blu-Ray discs use a laser wavelength which looks blue, hence the name.

Now that you know, never, ever, look into any laser light in an attempt to see the colour! Even the small lasers in recordable optical disc drives are powerful enough to burn the eye's retina, and cause instant and irreversible eye damage.

Optical drives are only considered "safe" because as part of normal operation, the laser unit itelf is securely enclosed, and the (powerful) laser light output is well protected from leaking out.

CD-ROM

CD-ROM stands for "Compact Disc, Read Only Memory". These look identical to an audio CD, but instead hold computer data. The standard CD-ROM format stores approximately 650 Megabytes (MB) of information.

DVD-ROM

In 1996, the DVD (Digital Versatile Disc) standard was created that allowed a far greater amount of information to be stored onto a single disc. Its primary purpose was to do for video what the Compact Disc did for audio: provide a consumer-friendly way to distribute high quality movies.

The DVD-ROM (Digital Versatile Disc, Read Only Memory) is the computer data version of the DVD standard. It allows approximately 4.7 GB (4700 MB) of data to be stored per disc -- approximately seven times more than a CD-ROM.

An extension to the standard DVD standard allows for two separate 'layers' of information to be stored onto the one disc. This scheme is known as dual layer DVD, and allows discs to store almost twice as much information (8.5 GB instead of 4.7 GB).

Blu-Ray Disc

The Blu-Ray Disc data format is most often associated with high-definition movies. However, it can also be used to store data: up to 25 GB per layer. Like DVD, these discs can also be dual layered, for a total capacity of 50 GB per disc.

Recordable Optical Formats

The non-recordable optical formats described above were intended to be mass-produced in factories: after a special source disc (called a "master") was created out of glass, it is used to make a mould from which plastic discs could be mass-produced.

Recordable compact discs are coated with a chemical which reacts to a high-power laser. 'Burnt' sections of the disc area are read like pits on a mass-produced disc.
Recordable compact discs are coated with a chemical which reacts to a high-power laser. 'Burnt' sections of the disc area are read like pits on a mass-produced disc.

In 1988 a variant of CD recording technology was developed: a way to create CDs using chemical, rather than mechanical means. A "blank" disc is created by treating a CD-shaped piece of plastic with a special chemical that was sensitive to laser light. This allowed CD drives equipped with special high-power lasers to hit the surface of the blank disc and "burn" the chemical equivalent of a pit onto the disc.

The first recordable optical formats were Write-Once, Read-Many (WORM). This means they could only be written to once. These standards are known as CD-R and DVD-R, for CD and DVD formats respectively. (The "R" stands for "Recordable"). Their capacity is the same as CD-ROM and DVD-ROM.

Other chemical treatments were later developed, including one whose effects could be reversed. This means that after data was written, the disc could be blanked out and used again. This prompted the development of new "RW" (Re-Writable) standards; CD-RW and DVD-RW.

A recordable version of the Blu-Ray format is also available, BD-R (recordable) and BD-RE (rewritable).

These are only the most common recordable optical formats; as processes were developed through the years, other schemes were also developed.

Among others, these include DVD-RAM, and the "plus" series of DVD-recordable formats (DVD+R and DVD+RW).

Performance Considerations

Being a rotating optical platter, optical drives share many intrinsic qualities of magnetic hard disks. However, there are some differences to how they operate; mainly due to the fact they are designed such that the recording media is ejectable.

Seek Time

Seek times are far inferior to magnetic hard disks - on the order of 100-150ms, instead of the 9-10ms of a hard disk.

This is a function of both the reduced density of the optical media, and the need for the read/write heads to constantly realign themselves - the removable media isn't securely bolted to the drive chassis like in a hard disk, and these variances need to be constantly accounted for.

Rotational Latency

Seek times of optical media are significantly slower than magnetic hard disks; as the disc is designed to be removable, they cannot be held as securely in the drive unit, and thus rotational speeds must be reduced.

The rotational speed of a DVD disc is between 630 and 1530rpm. This compares poorly to a desktop hard disk's typical rotational speed of 7200rpm. The reason why a DVD's (and most optical media's) rotational velocity varies is due to the disc being spun at a constant linear velocity (CLV). What this means is that the data passing under the read/write head is at a constant speed, regardless of whether it is at the inner or outer surface of the disk.

Data Fragmentation

As optical media is either mastered (non-recordable formats), or recorded in sessions rather than individual files (recordable formats), data fragmentation is not an issue for optical media.

Transfer Rate

With the above in mind, sustained transfer rates for optical media (assuming a fairly sequential work load) tend to be much more consistent than for magnetic media. This is a by-product of the rotational speed of the disk being determined by a constant linear velocity approach.

This is appropriate, as one of the primary uses for optical media is for recording and replaying of audio (Audio CD) and video (DVD and Blu-ray) content. Since these are rendered at a specific and standardised bit rate, and thus the data storage standards and hardware is optimised around what is necessary to maintain that data transfer rate.

In general purpose computing applications, faster is generally always better, and this is why optical drives are rated by how much faster than the standard they can run. For example, an 8X DVD writer can write a DVD disc, 8 times faster than the transfer rate standard specified in the original DVD specification.