OVERVIEW OF OPTICAL STORAGE SYSTEMS

Key Performance Parameters of Optical Disk Systems

The storage media of most optical storage systems in production today are in the form of a rotating disk. Figure 3.1 illustrates a typical optical disk system. In general the disks are preformatted using grooves and lands (tracks) to enable positioning an optical pickup and recording head to access information on the disk. A focused laser beam emanating from the optical head records information on the media as a change in the material characteristics. To record a bit, the laser generates a small spot on the media that modulates the phase, intensity, polarization, or reflectivity of a readout optical beam; that beam is subsequently "read" by a detector in the optical head. Drive motors and servo systems rotate and position the disk media and the pickup head, thus controlling the position of the head with respect to data tracks on the disk. Additional peripheral electronics are used for control and for data acquisition, encoding, and decoding. As for all data storage systems, optical disk systems are characterized by their storage capacity, data transfer rate, access time, and cost.

Storage capacity

The storage capacity of an optical storage system is a direct function of spot size (minimum dimensions of a stored bit) and the geometrical dimensions of the media. A good metric to measure the efficiency in using the storage area is the areal density (MB/sq. in.). Areal density is governed by the resolution of the media and by the numerical aperture of the optics and the wavelength of the laser in the optical head used for recording and readout. Areal density can be limited by how well the head can be positioned over the tracks; this is measured by the track density (tracks/in.). In addition, areal density can be limited by how closely the optical transitions can be spaced; this is measured by the linear density (bits/in.).


Fig. 3.1. Key components of an optical disk system.

Data transfer rate

The data transfer rate of an optical storage system is a critical parameter in applications where long data streams must be stored or retrieved, such as for image storage or backup. Data transfer rate is a combination of the linear density and the rotational speed of the drive. It is mostly governed by the optical power available, the speed of the pickup head servo controllers, and the tolerance of the media to high centrifugal forces.

Access time

The access time of an optical storage system is a critical parameter in computing applications such as transaction processing; it represents how fast a data location can be accessed on the disk. It is mostly governed by the latency of the head movements and is proportional to the weight of the pickup head and the rotation speed of the disk.

Cost

The cost of an optical storage system is a parameter that can be subdivided into the drive cost and the media cost. Cost strongly depends on the number of units produced, the automation techniques used during assembly, and component yields. Optical storage R&D typically concentrates on the following efforts: reducing spot size using lower-wavelength light sources; reducing the weight of optical pickup heads using holographic components; increasing rotation speeds using larger optical power lasers; improving the efficiency of error correction codes; and increasing the speed of the servo systems. Equally active R&D efforts, especially in Japan, are focused on developing new manufacturing techniques to minimize component and assembly costs.

Optical Disk Formats

Depending on the access times required by given applications, optical disk products come in two different formats: the compact disk (CD) format used for entertainment systems (audio, photo, or digital video disk applications), and the standard or banded format used for information processing or computing applications.

CD format

In the optical disk CD format, information is recorded in a spiral while the disk turns at a constant linear velocity. The standard disk diameter used is 12 cm, which offers a typical capacity of 650 MB with a seek time (access time) in the order of 300 ms and data rate of about 10 kB/s. A minidisk format is currently being adopted in some Sony products that use 6 cm disks providing 140 MB capacity. Various types of products belong to the CD family, including CD recordable (CD-R) products, which are the write-once, read-many (WORM) version of standard CDs; the CD-E erasable products, which are to appear shortly in the market; the Photo-CD systems, which were first marketed by Kodak for storing images; and video CDs, which may become available over the next two years. Several standards for video disk systems are presently being put forward, including the double-sided video disk (DVD) standard proposed by Toshiba and the double-layer format proposed by Sony. Major improvements in CD technology are expected to take place within the next few years.

Standard format

The access time achieved by the CD format is too slow for use in computing applications. To shorten access times, a standard format is commonly used in magnetic as well as optical disk systems, where the disk turns at a constant angular velocity and data is recorded on concentric tracks. Whether the inner or outer tracks are read, the disk's speed of rotation remains constant, allowing for faster access times; however, this format wastes valuable disk space on the outer tracks, because it requires a constant number of bits per track, limited by the number of bits that can be supported by the innermost track. To eliminate this waste, a "banded" format is now used where tracks of similar length are grouped in bands, allowing the outer bands to support a much larger number of bits than the inner bands. This, however, requires different channel codes for the different bands in order to achieve similar bit error rates over the bands.

In the standard format, 12 in., 5.25 in., and 3.5 in. disk diameters are commercially available, and 14 in. and 2.5 in. disk diameters are being investigated. The 12 in. products (mostly WORM) provide high-capacity solutions on the order of 7 GB on a single platter for storage of large databases, achieving areal densities exceeding 500 MB/sq. in. The 5.25 in. disks are most commonly used today and provide data capacities of 2 GB per disk, seek times on the order of 35 to 40 ms, and data rates on the order of 2 to 5 MB/s. They achieve an areal density of 380 MB/sq. in., and are cost-competitive at $200/disk or $0.10/MB. The 3.5 in. disks presently provide one-eighth of the capacity of 5.25 in. disks, reaching only 128 MB, but for the low cost of $30 per disk ($0.25/MB). Recently, a new generation of (2X) products have been released that provide a 230 MB capacity.(Endnote 1)

Optical Storage in Hierarchical Memory Systems

During the past fifty years, many memory technologies have been developed. Despite intense competition, several widely different approaches are currently in use: magnetic and optical tape; hard disks, floppy disks, and disk stacks (Bell 1983); and both electronic static random-access memory (SRAM) (Maes at al. 1989) and dynamic random-access memory (DRAM) (Singer 1993). There are also several newer technologies now available, such as the solid-state disk (Sugiura, Morita, and Nagasawa 1991), the Flash Erasable Electrically Programmable Read-Only Memory (EEPROM) (Kuki 1992), and the Redundant Array of Inexpensive Disks (RAID) (Velvet 1993) systems.

This proliferation of technologies exists because each technology has different strengths and weaknesses in terms of its capacity, access time, data transfer rate, storage persistence time, and cost per megabyte. No single technology can achieve maximum performance in all these characteristics at once; modern computing systems use a hierarchy of memories rather than a single type. The memory hierarchy approach utilizes the strong points of each technology to create an effective memory system that maximizes overall computer performance given a particular cost.

Hierarchy levels

In standard sequential computer architecture there are three major levels of the storage hierarchy: primary, secondary, and tertiary.

Primary memories (cache and main). Primary memories are currently implemented in silicon and can be classified as cache memory (as local storage within the processing chip) and main memory (as RAM and DRAM chips located on the same board). The access times of primary memories are comparable to the microprocessor clock cycle, but their data capacity is limited (10 to 100 MB for main), although it has been doubling every year.

Secondary memories. Secondary memories, such as magnetic or optical disk drives, have significantly increased capacity (into gigabytes), with significantly lower cost per megabyte, but the access times are on the order of 10 to 40 ms.

Tertiary (archival) memories. Tertiary memories store huge amounts of data (into terabytes, or 10 12 bytes), but the time to access the data is on the order of minutes to hours. Presently, archival data storage systems require large installations based on disk farms and tapes, often operated off line. Archival storage does not necessarily require many write operations, and write-once, read-many (WORM) systems are acceptable. Despite having the lowest cost per megabyte, archival storage is typically the most expensive single element of modern supercomputer installations.

Storage capacity versus access time

Figure 3.2 compares the various components of the memory hierarchy of commercial systems available in 1993. This figure shows capacity and access time for currently available memory systems, and it depicts the trade-off between short access times and high capacity. The figure also shows that for desktop computing applications, optical storage devices do not compete well with magnetic storage systems, due to their slower access times. This limitation prevents optical storage systems from being used for personal computer hard drives and has restricted seriously the application of optical systems during the last decade.


Fig. 3.2. Comparison of capacity and random access time of computer memory systems. The general trend is that higher capacities are obtained at the expense of longer access times (Call/Recall, Inc.).

Magnetic systems. Areal density of magnetic systems is governed by the minimum switchable area of a magnetic domain. The size of these domains is governed by the dimensions of the magnetic heads and their distance to the active media. These domains can be made quite small, since the magnetic heads can be miniaturized and are "flown" right against the media (approximately 50 nm above). The access time of magnetic disk devices is in general shorter than optical disk systems by about one order of magnitude, because of the low inertia of these miniature heads and the faster rotation speed of the media. This same advantage, however, is also associated with two of the main disadvantages of magnetic storage: head crashes and nonremovability. It should be pointed out, however, that some magnetic disk products provide removability at the expense of longer access times.

Optical systems. Up to recently, interest in optical storage systems was restricted to use for very large storage systems and backup systems, because of their robustness and removability. Optical storage for very large storage devices employing interchangeable and recordable media in automatic "jukeboxes" is a market traditionally outside the range of magnetic disk drives but directly in competition with magnetic tapes. The advantage of optical systems for this market is that they have much shorter access times than tapes.

Storage capacity versus cost

The market direction for optical disk systems can be anticipated by examining cost per megabit as a function of system capacity, as shown in Figure 3.3. The generally decreasing trend seen in this graph indicates that as capacity increases, cost per megabit decreases. The solid lines show total system cost for the three storage system types. These lines indicate that the total cost of secondary and tertiary memory systems far exceeds the cost of primary memory.


Fig. 3.3. Comparison of computer memory systems in terms of cost and capacity. The strong linear relationship shows that as capacity increases, cost per megabit decreases, but not in the same proportion. The result is that high-capacity systems have a much higher total system cost (Call/Recall Inc.).

This graph also shows that by the end of 1993, some optical storage products provided lower cost per megabit than magnetic storage systems, and in addition, they offered media removability. This makes optical storage systems attractive for the very-high-capacity tertiary storage systems and potentially attractive for personal computer (PC) backup systems. Tertiary optical systems face fierce competition from both magnetic RAID systems (arrays of low-cost magnetic disk drives connected in parallel) and magnetic tape systems. The optical backup systems for PCs start suffering from competition from newly released magnetic products such as the Iomega Zip TM drives ($200/drive), which provide 100 megabyte removable floppy disk-like media at very low cost ($20/disk, $0.02/Mbit), yet provide similar access times and data transfer rates as optical disks.

Benefits and applications of optical storage

In the early 1990s, another direction opened up for optical storage devices with CD format. Due to their low-cost replication capability, high capacity, robustness, and removability, optical CD-ROM systems have become competitive with magnetic floppy disks for applications such as software distribution and home multimedia applications. The success of CD-ROM technology in the consumer market has allowed for the cost of optoelectronic components such as CD lasers to drop sharply over the last few years, paving the way for new applications and new optical storage systems. It is expected that CD systems will remain essential for the wide commercial acceptance of optical storage systems.

In sum, the attractive unique features of optical storage systems are their higher capacity per disk, removability, mass replicability, and long memory persistence for archival applications. They are most commonly used for software distribution, backup memory for personal computers and workstations, external memory for some mainframes, and as large- capacity off-line memory. Key applications include text and graphics filing, statistical data and ledger storage, public and historical database storage, and possibly as replacement for paper. New applications and markets opening to optical storage systems as their prices are dropping include home multimedia, multimedia servers, high-definition television and digital video disks, and massive storage systems.


Published: February 1996; WTEC Hyper-Librarian