Section 9.4. Optical Drives
9.4. Optical DrivesOptical drives fit a niche in between the inexpensive nature of tape and the throughput and flexibility of disk. They excel in the areas of reliability, flexibility, duty cycle, removability, and time-to-data. Their challenges are in the areas of throughput, capacity, and cost. Optical drives offer the removability of tape, but their capacities and throughput rates have generally lagged behind those of tape. Their greatest advantages over tape have been their very quick time-to-data rates and their long-term reliability. Although inexpensive disk is now easily the winner in the time-to-data category, most disk systems are not designed to be portable. Therefore, optical systems are an easy choice for environments that require very quick time-to-data rates along with removability. In addition, if you need to reliably store data for long periods of time, optical is the clear winner. (Some disk units are even competing for that category as well, but many feel it's a stretch. We'll see what happens.) If you need a removable media with random accessibility that can reliably store data for long periods of time, optical tape drives are hard to beat.
This section is divided into two main subsections: optical recording methods and optical formats. Throughout, a recording method refers to how the data is physically represented on disk, and a recording format refers to what type of drive may use these recording formats. For example, the most common recording method is the phase change method. There are several recording formats that use this, such as CD-RW, DVD-RW, DVD-RAM, and DVD+RW. 9.4.1. Optical Recording MethodsMost people understand that traditional disk drives record digital (binary) data by polarizing sections of the disk. With traditional optical recording methods, pits (or holes) in the land (or surface) of the disk represent the binary data. Historically, the land was the flat surface of the disk, and the pits were actual holes burned into the land. When the laser reads the disk, pits don't reflect as much light as the land, and this is translated into binary data. Many newer recording technologies do not produce actual pits. The land is composed of material that is sensitive to high-powered lasers. When a laser is applied to a certain area, it changes the reflective properties of that area so that it appears to be a pit in the land. (Although they aren't actually pits, they still are referred to as such.) The magneto-optical recording method is a hybrid of magnetic and optical technologies, thus its name. Binary data is represented by realigned magnetic particles, as with a traditional disk or tape drive. A laser is used during the recording process, and a laser is used when reading the drive as well. (More on this later.) 9.4.1.1. Magneto-optical recording methodWith magneto-optical (or MO), the magnetic recording layer is heated by a laser, which makes it easier to polarize. The data then is recorded using traditional magnetic recording techniques, although the heated nature of the recording surface allows precise control over the magnetized areas. Once the recording layer cools, it is less susceptible to magnetic degradation. An MO disk is erased by passing the high-powered laser over it again and writing all zeros to the disk. This two-step operation usually requires two passes to complete, but some MO drives have figured out how to do it in one pass. MO drives range in size from a few hundred megabytes to 9.1 gigabytes. MO media traditionally has been permanently contained within a cartridge that looks similar to a very big 3.5-inch floppy. (You do remember floppies, right?) At a time when CDs offered only a few hundred megabytes, and affordable DVD writers didn't exist, MO drives offered the first affordable "optical" recording method for larger amounts of data. Although it was actually a magnetic recording, it stored data longer than traditional magnetic recordings because the media had to be heated to change its magnetic properties. The result was that MO ruled the archive world for quite a few years. However, the introduction of the phase change recording method changed all that. 9.4.1.2. Phase change recording methodPhase change recording is the only truly optical recording method, as compared to MO, which is a mix of both optical and magnetic recording. Disks recorded with this method contain a special recording layer that changes between a crystalline and amorphous state when heated by a laser, which is why it is called "phase change." The amorphous state is less reflective than the crystalline state, and sections of the disk that are in the amorphous state appear as pits when the disk is read. A higher-powered laser returns the entire disk to the crystalline state, thus erasing the data. As of this writing, it is used in all recordable and rewritable CD, DVD, and UDO drives. The phase change recording method has some advantages over the MO recording method, starting with the fact that a disk can be erased in a single pass. Secondly, the nature of laser recording allows for lasers with smaller and smaller diameters, whereas the lasers themselves become more powerful. We started with 780 nm pits with CDs and followed with DVDs with 630 nm pits. Now the blue lasers used by UDO drives are only 405 nm wide. The small width of the lasers allow for tighter tracks with bits that are closer together, allowing for more data on the disk. Couple that with higher-powered lasers that can read and write multiple layers, and you can understand how we've gone from 650 MB to 30 GB on the same diameter disk. The result is that, at this point, devices using phase change recording have really taken over the market. 9.4.1.3. Dye polymer recording methodThis recording method has been adopted by a few manufacturers and uses a special die that bubbles when heated. The bubbled areas have different reflective properties than the nonbubbled areas, and appear as pits when read by the laser. Again, a higher-powered laser resets the dye to its original state, thus erasing the data. 9.4.1.4. WORM recording methodsSlight variations to the recording methods just discussed result in write-once-read-many recordings that create an archive that cannot be deleted, changed, or overwritten. A number of companies use WORM recording methods to create archives that can be used as proof of things that happened in the past, because you can prove that they haven't been changed. CD-R, DVD-R, DVD+R, and UDO all use a variation of the phase change recording method to create WORM disks. There are also WORM variations of the MO recording method. There's even WORM tape. For a recording to be WORM, the media must be designed so that it can be changed only once. For example, the popular ablative recording method uses a tellurium alloy as a recording layer. This alloy has a low enough melting point that the high-power laser actually does make physical pits in it. Once these pits are in the disk, they cannot be removed. 9.4.2. Optical Recording FormatsThe following is an overview of recordable and rewritable optical formats. They will be covered as generally as possible, because the details change every time you look at them. 9.4.2.1. CD recording formatsRecordable and rewritable CDs are quite popular. They have a small capacity (about 700 MB) and a relatively slow transfer rate (12 MBps), but everyone can read a CD these days. Therefore, they're useful for data interchange if nothing else. Here are the two types of CD recorders and how they work:
9.4.2.2. DVD recording formatsDepending on who you ask, DVD stands for digital versatile disk or digital video disk. DVD has gained a lot of popularity in recent years, and it brings a lot to the table. There are also a lot of automation vendors that offer rewritable DVD libraries. The different recordable formats can fit between 2.6 GB and 4.7 GB per side, for a total of 9.4 GB for a dual-layer, dual-sided disk. The transfer rate (520 MBps) is still very slow compared to modern tape drives, but most recordable DVD formats support random access, so they have a very quick time-to-data. At one time, the user was required to carefully select a format for the long term. There are two WORM formats (DVD-R and DVD+R) and three rewritable formats (DVD-RW, DVD+RW, and DVD-RAM). At this point, however, most recordable DVD drives can read or write any of the five competing formats. The DVD formats, described below, are sponsored by two different groups: The DVD Forum (www.dvdforum.com) and the DVD+RW alliance (www.dvdrw.com). The DVD Forum was the original alliance of DVD manufacturers, but their DVD drives and media are no more or less official than those provided by the DVD+RW alliance. Drives with a hyphen or dash in their name (DVD-RAM, DVD-R, and DVD-RW) come from the DVD Forum, and those with a + in their name (DVD+R, DVD+RW) come from the DVD+RW Alliance.
Unix Backup & Recovery had a compatibility chart for the various DVD formats. However, I found through interviews and my own experience that your mileage will vary greatly. Generally speaking, the DVD-R format is the most compatible with other players. I know that's what I burn when I want to make sure the other person can read the DVD. Other people have told me that they haven't had near the problems that I have had with incompatibility. That's why I say your mileage will vary. Do your own testing to be sure. 9.4.2.3. Magneto-optical recording formatMO drives use the MO recording method and are readily available from a number of vendors. There is also a big line of automated libraries that support MO drives and media. This level of automation, combined with its low cost, used to make MO the choice for nearline environments. The challenges of MO, including a limited capacity (9.1 GB) and a multipass rewrite, caused people to start to look elsewhere. As of this writing, it appears that DVD and UDO have gained significant ground in this area. 9.4.2.4. UDO recording formatUltra Density Optical, or UDO, is a 5.25-inch rewritable optical disk designed by Plasmon that also supports both rewritable and WORM media. As of this writing, it fits 30 GB on a single cartridge using blue lasers and phase change recording. Cartridges that store 60 GB and 120 GB are on the way. While UDO is similar to CD and DVD drives designed for the consumer, it was designed with the data center in mind, with more expensive, heartier components than you can use in a consumer-grade appliance. UDO also uses an eight-layer disk, which is twice the number of layers present in a typical consumer-grade DVD. |
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