Section 9.3. Tape Drives


9.3. Tape Drives

Here's some helpful information about tape drives.

9.3.1. Tape Drives Must Be Streamed

Modern tape drives do not generally respond well to different data rates. That is, if a tape drive was designed to read or write data at 120 MBps, then it will typically perform well only if it's reading or writing at 120 MBps, with some exceptions that are covered later in this chapter in the section "Variable Speed Tape Drives." Most tape drives today can actually only write data at their rated speed. If you send it 120 MBps, it will write at 120 MBps. However, if you send that same drive at 60 MBps, it will spend 50 percent of its time writing at 120 MBps and the other 50 percent of its time preparing to write at 120 MBps.

Here's how it works. You start sending the data to the drive, where it first goes to the drive's buffer. The tape drive starts spinning up its mechanics, preparing to write the data from the buffer to the tape at 120 MBps (if it's a 120 MBps drive). Once the buffer is full, the tape drive starts reading data from the buffer and writing it to the tape. If you're streaming the drive, you're filling the buffer with new data as fast as the drive is emptying the buffer as it writes data to tape.

However, if you're not filling the buffer as fast as it's being emptied, at some point, the tape drive looks at the buffer and it's not full. The tape is still moving, of course, because it needs to be moving to write databut now there's no more data to write. The tape drive must stop, rewind the tape, move it back to a position before it stopped writing, and then wait for the buffer to fill up with data again. This is called repositioning, and every reposition takes a finite amount of time, sometimes as much as a few seconds. When the buffer is full again, the drive starts moving the tape again, and the buffer gets emptied again.

If the buffer is constantly full of data, and the tape drive never has to reposition, we say that the tape drive is streaming. If you are sending anything less than the drive's target throughput rate, the drive is repositioning. If you're sending a stream of data that's less than, but close to the target throughput rate, it's repositioning occasionally. The slower the data rate is in comparison to the target rate, the more the drive is repositioning. If the drive is repositioning a lot, the tape is being constantly moved back and forth across the read/write head like a shoe-shine rag across a dress shoe, which is why this scenario is called shoe-shining.

It usually takes longer to reposition the tape than it does to empty the buffer. Therefore, once a reposition has started, it doesn't matter how quickly you can fill up the buffer; the drive has to finish its reposition before it can start writing and empty the buffer again. Meanwhile, the buffer might be full, but it isn't being emptied. This means that the incoming data must be told to wait while the drive prepares to empty the buffer. The more often this happens, the less the drive is able to keep up with the incoming data rate. This is why a tape drive that is not streaming may actually write data at a slower rate than the incoming data rate; the drive is spending all of its time repositioning, and very little of its time writing data.

9.3.2. Compression Makes It Harder to Stream Drives

Although you should not use compression rates when comparing different backup drives, it is important to understand the role that compression plays in determining actual throughput. As mentioned in the section on compression earlier in this chapter, a lot of people do not realize that compression increases the effective throughput rate of your drive by the same ratio it increases a drive's effective storage capacity. In the previous section, I mentioned that a 120 MBps tape drive should be sent data at the rate of 120 MBps if you expect it to stream. This is called its target throughput rate. However, if the data going to a 120 MBps drive is being compressed 2 to 1, its effective target throughput rate is actually 240 MBps. A drive is streaming only if it's writing data at or near its effective target throughput rate.

You need to determine the effective target throughput rate of the drives in your environment. To do this, you must determine what compression ratio you're actually getting, then multiply that ratio times your drive's target throughput rate (e.g., 120 MBps) to get your effective throughput rate (e.g., 240 MBps). To determine your compression ratio, you need to determine how much data is being written to your full tapes. Most backup products write data to a tape until they hit the PEOT mark. This means that if you divide the average size of a full tape by the native capacity of a tape, you will end up with your actual average compression ratio. For example, if the native capacity of your tapes is 400 GB, and you're fitting an average of 600 GB of data on each full tape, you're getting an average compression ratio of 1.5 to 1. Therefore, your 120 MBps tape drive is actually a 180 MBps tape drive.

9.3.3. Variable Speed Tape Drives

The previous section explained how tape drives do not handle varying data rates very well because they can write at only one speed. As mentioned previously, this is really a function of the need to have the recording head move across the media very quickly to obtain a high signal-to-noise ratio.

Having said that, tape drive vendors had gotten tired of hearing about this old complaint, and they were genuinely worried that the problem was going to get worse and worse as tape drive speeds got faster and faster. The faster drives got, the harder it would be to stream them, and the better the chance they would shoe-shine. Yet the nature of the market required them to come out with faster and faster tape drives.

So a few bright vendors started to realize that if they came out with a faster drive that could also go slow, they'd have something truly special. Therefore, some drives are now able to step down their speeds to keep up with slower data rates. As of this writing, there are drives that can go as slow as 1/2 of the original native transfer rate; in other words, a 100 MBps drive that can also run at 50 MBps without shoe-shining.

Some vendors are claiming that variable speed tape drives make shoe-shining a thing of the past. This is absolutely not true.


Variable speed tape drives can still suffer from shoe-shining. Suppose a tape drive has a native speed of 120 MBps, and your data is compressing at 1.5 to 1, turning that drive into a 180 MBps tape drive. A variable speed tape drive can typically slow down to 50 percent of its original speed, allowing the drive in our example to become a 90 MBps tape drive after compression. If you are supplying data slower than 90 MBps, then it is not streaming; it is shoe-shining.

9.3.4. Helical and Linear Tape Drives Are Different

It's generally held that helical scan tape drives suffer less than linear tape drives when it comes to slow incoming data rates. A review of the differences between these technologies will help explain why.

One of the best ways to illustrate the difference between helical scan and linear recording technologies is to look at a non-hi-fi VCR, because it actually incorporates both technologies and illustrates an important point. Do you remember VCRs before they all went hi-fi? Did you ever record and watch a movie on a non-hi-fi VCR using the extended play (EP) setting? When you played that tape, it sounded horrible.

This analogy worked much better 510 years ago when not all VCRs were hi-fi. Some readers may have to go ask their parents what a non-hi-fi VCR was. If you've never had a non-hi-fi VCR, you'll just have to trust me. Recording a movie on EP sounded like garbage!


Yet if you record the same movie on the same setting with a hi-fi VCR, the audio sounds fine. Have you ever wondered why?

Look at Figure 9-2. A VCR's tape is brought out of the cartridge and wrapped around a rotating drum. As you can see in Figure 9-3, the drum is angled slightly and has recording heads on its side. (The rectangle sitting at an angle in Figure 9-3 represents the angled drum with its rotating recording heads.) As the tape is pulled slowly around the drum, the diagonally positioned recording heads write "stripes" of video data diagonally across the tape, as can be seen in the bottom of Figure 9-3. Although the tape is moving very slowly around the drum, the drum is spinning very fast. This means that the recording heads on the edge of the drum actually are moving across the tape very quickly, resulting in a good quality video signal.

Figure 9-2. VCR tape path


Figure 9-3. Helical scan recording


The drum spins at 1800 rpm, or 30 revolutions per second, with one head on each side of the drum. This means that the recording heads are writing a stripe of data 60 times each second. Each one of these stripes contains half of an interlaced video frame. A synchronization signal also is written along the edge of the tape that keeps the tape in sync with the spinning recording heads. The VCR interlaces these images into what you see as full-motion video.

In a non-hi-fi VCR, the tape also passes by a stationary audio head that records a linear audio signal along the edge of the tape; this is very similar to how an audiocassette player works. You can see the stationary audio recording head in Figure 9-3. A hi-fi VCR has this same stationary head for backward-compatibility reasons, but it also has audio heads on the spinning drum. This means that it records audio tracks as diagonal stripes alongside the video data, as can be seen in Figure 9-4. The audio recording heads in a hi-fi VCR are moving across the tape at a high speed, whether you are recording in EP or standard play (SP) mode. This is why a hi-fi VCR can record a good audio signal regardless of the speed at which the tape is moving around the drum.

Figure 9-4. A section of videotape


The result is a videotape that looks like the one in Figure 9-4. The video tracks are recorded in diagonal stripes across the tape. In a non-hi-fi VCR, the audio track is recorded slowly, in a linear fashion, along the bottom of the tape as the tape passes by the stationary recording head. A hi-fi VCR also records the audio this way but also records it in quick, diagonal stripes parallel to the video tracks.

Remember that in a non-hi-fi VCR, the video heads move very quickly across the tape, but the audio heads do not. The result is a good-quality video signal, but a poor-quality audio signal. However, when a hi-fi VCR records audio, it records it just like it records the videoin rapid diagonal stripes across the tape. This results in high-quality audio and video signals. This shows how, in order to record a high-quality signal to tape, the recording head must be moved across the media very quickly. This is important because in a data drive, the quality of the signal is everything.

This in-depth explanation of how VCRs work also explains the difference between helical scan and linear recording technologies. Helical scan drives record data just like a VCR records video, by wrapping the tape around a spinning drum with recording heads attached to it. Linear tape drives move the tape quickly across a stationary recording head. Let us look at these two types of drives in more detail.

As seen in Figure 9-3, a helical drive pulls the tape out of the drive and wraps it around a spinning drum that is turned on a slight angle. On the side of the spinning drum are recording heads that write diagonal stripes across the tape. This allows the tape to move very slowly around the spinning drum, while the recording heads can move across the tape's surface very quickly. This is exactly the same way a VCR records the video signal.

The downside of such a device is that the tape must be wrapped all the way around this spinning drum. Advocates of linear recording technology say that this puts undue stress on the tape. Helical scan manufacturers say that they have changed the way the tape is pulled around the drive in a way that reduces the stress on the tape. Helical scan technology is used in several drives, such as 8 mm, AIT, DAT/DDS, DTF, DTS, and SAIT.

A tape drive that uses linear recording technology pulls the tape very quickly across a fixed recording head. Remember from the lesson about VCRs that the recording head must move very quickly past the tape. Since the recording head is stationary, this requires moving the tape at a very high speed, as much as hundreds of inches per second. One of the popular drives that uses linear recording technology is Linear Tape Open (LTO); its tape path is illustrated in Figure 9-5.

Figure 9-5. Linear recording technology


Most modern linear tape drives, including LTO, use an enhanced linear technology called linear serpentine. A drive using the linear serpentine recording method records several stripes of data across the tape from one end to the other. The head then moves slightly up or down and writes another several stripes of data in the reverse direction. Depending on the model of the drive, it may do this several times before it uses up the entire recording surface. Linear technology is also used in a number of tape drives, including DLT, LTO, and all of the drives manufactured by IBM and Sun as of this writing.

Now that you understand the differences between these two technologies, you should be able to see why many feel that helical scan tape drives might suffer shoe-shining less than linear tape drives. A linear tape drive is moving the tape very fast, where a helical scan drive is moving it very slow. A linear tape drive with an empty buffer is going to move past several hundred inches of tape before it "realizes" that the buffer is empty and it begins to reposition. The tape in a helical scan drive, however, isn't moving nearly as fast, so it's won't move that far before it realizes it needs to rewind, and it doesn't have to rewind very far before it gets back to the point it needs to be before it starts writing again. Some time will be associated with syncing up to the sync signal on the bottom of the tape that keeps the heads in sync with the diagonal stripes of data on the tape, but that will probably be faster than what's happening in a linear tape drive.

9.3.5. Cartridges Versus Cassettes

Although many people use the term "cartridge" to refer to any type of tape, cartridges are actually single-spool tapes, such as a DLT, LTO, or SAIT drive. A cassette contains two spools within the tape, such as an AIT or DDS drive.

The reason for discussing this is to explain one primary difference between the way the two types of tape work. A single-spool cartridge does not have a take-up reel inside the cartridge itself. The take-up reel is inside the drive. That means that with single-spool cartridges, the entire tape is pulled out of the cartridge and wrapped around the drive's internal take-up reel.

A cassette's tape, however, effectively remains inside the cartridge. Most technologies pull a certain amount of the tape outside of the cassette at a time, but the bulk of the tape remains inside the cassette. There are a few technologies that can use a cassette without pulling any of the tape outside of the cassette.

Must've Saved a Bundle

A backup service provider that supported remote backups for several customers scheduled tape pick-ups/drop-offs to occur not at the datacenter, but at a central location. Tapes were transported between the central location and the datacenters in the back of a backup operator's station wagon, sometimes sitting in the back of a hot car for several hours.

Kevin Suttle


9.3.6. Midrange Tape Drive Types

This section briefly covers what the industry tends to call midrange tape drives. A midrange tape drive is typically something that would be considered expensive to a home user but is not a high-end specialized tape drive, such as those found in the black boxes of planes. The tape drives covered in this section currently range from under a thousand dollars to tens of thousands of dollars.

We do not list the capacity of the drives listed here because they change so frequently. You can easily obtain this information from the Web.


The information I have included is very general and often historical, and is offered mainly to assist you in differentiating among the different types of drives. The drives are listed in alphabetical order as much as possible so as not to show any preference for any particular drive. Some of the drives covered here are either brand new to the market or not even released as of this writing.

Some of the drives covered in this chapter have been end-of-lifed by their manufacturers, and I considered dropping them from the book. However, the truly budget-conscious customer just might be buying one of these end-of-lifed drives on the used market. I left them in just for that reader.


9.3.6.1. 3480 (end-of-lifed)

The IBM 3480 drive family has been around awhile; uses a half-inch 3480 cartridge; and includes the 3480, 3490, and 3490E. Although today these drives are made by a number of different manufacturers, they originally were created for IBM mainframes. These drives, therefore, have the longest history of stability and reliability of any drive being sold in the open-systems market today. They are rather slow and small in capacity compared to other tape drives, but they have a relatively quick load and access time.

9.3.6.2. 3590

The IBM 3590 was the successor of the 3480 family, with larger tape capacities and additional speed. It has its own media typea half-inch 3590 cartridge.

9.3.6.3. 3592

The IBM 3592 was intended to replace the 3590. It uses its own half-inch 3592 cartridge and offers faster transfer rates and higher capacities than its competitors. There are actually two types of 3592 media. One is less expensive, while the other offers quicker access to data.

9.3.6.4. TS1120

The IBM TS1120 uses 3592 media and offers extremely fast transfer rates and very large capacities.

9.3.6.5. 3570 drive (a.k.a. Magstar MP)

The IBM 3570 cartridge, with its trapezoidal shape, is a completely different form factor than just about any cartridge out there. It was the first midrange tape drive that could mount midpoint; it also never leaves the cartridge. (In the tradition of borrowing old technologies, this one reminds me of an old cassette tape. You remember how pinchers were inserted into the tape, and the rollers pulled the tape along without removing it from the cartridge? This mechanism is reminiscent of that.) The tape has a relatively slow transfer rate, but transfer rate is not the most important factor in the market that this tape is aimed at. It is a potential nearline solution, because it has a total time-to-data of less than 30 secondsnot that much slower than the optical drives that were available at the time it was released.

9.3.6.6. 8 mm (8x0x) drives (end-of-lifed)

This family of drives was originally made only by Exabyte, although other manufacturers eventually made them as well. (This category does not include the AIT or the Mammoth drives, which are covered separately.) The first drive in this family was the 8200, followed by the 8500 and 8505. These drives have a poor reliability history. Insiders will tell you that it is because the mechanisms that go into these drives are the same mechanisms that go into Sony 8 mm camcorders. At one time, they actually were made on the same assembly line.

These drives were so much like camcorders that back in the day, we actually used consumer-grade video tapes as backup tapes. Although we cursed our cheap purchasing department, they actually worked just fine.


9.3.6.7. 9840 drives

This line of Sun/StorageTek drives uses a half-inch 9840 cartridge and includes the 9840A, B, and C. At first glance, they have smaller capacities and throughput rates than other drives listed in this chapter, and yet they cost more. There are two reasons for this. The first is that, like the IBM 3xx0 drives, these drives were designed for mainframe use and are intended for a 100 percent duty cycle. The second reason is that they tend to have very quick time-to-data rates. If you need a very quick-acting tape drive that has a 100 percent duty cycle, this drive is for you.

9.3.6.8. 9940 drives

The 9940 drives are also from Sun/StorageTek and use a half-inch 9940 cartridge. They offer more capacity and throughput than the previous 9840 generation.

9.3.6.9. T10000 drives

The next generation of drives from Sun/StorageTek uses a half-inch 10000 cartridge that offers more capacity and throughput than the 9940 family.

9.3.6.10. AIT drive

The AIT drive is Sony's attempt to completely reengineer the 8 mm drive. These drives fit into a 3.5-inch half-height form factor. The tapes are 8 mm high, but that's where the similarity with the old 8xx0 drives ends. Sony invented a new type of media just for the drive, called Advanced Metal Evaporative (AME), which consists of an evaporated metal recording layer covered by a protective layer and lubricant. This new media type reportedly has superior recording and magnetic retention capabilities over the standard magnetic particle tape. The tape also contains an EEPROM called Memory In Cassette (MIC). This EEPROM contains historical information about the tape, and potentially could be used to partition the tape into multiple logical volumes, although no vendors have taken advantage of this functionality. The AIT drive cannot read traditional 8 mm tapes. The drive did deliver on its promise of capacity and throughput, at a relatively low price point, and was designed to compete with the original DLT market.

9.3.6.11. DDS drive

Originally put out by HP, the Digital Data Storage (DDS) drive borrowed the format from the DAT market. Just for the record, it is not proper to call a DDS drive a DAT drive, because DAT refers to digital audio tape. Many people (and some vendors) still call DDS drives "DAT drives," even though it's about the same as calling an 8 mm drive a camcorder. Very few people will notice or care if you make this common mistake. It probably would be easier to get people to stop saying "PIN number." DDS drives are one of the least expensive and slowest drives in the open-systems market. They work, they're inexpensivebut they are slow. They're also quite popular, as they were the only midrange drive under $1,000 for a long time.

9.3.6.12. DLT drives (end-of-lifed)

DLT stands for Digital Linear Tape, and these drives were originally developed by Digital Corp., based on its TK-50 and TK-70 lines. The company kept the same basic media format and redesigned the drive that used it. (The first DLT drives were actually able to read the old TK tapes.) Two years later, it improved the design with what would come to be known as the DLT 2000, with twice the capacity and 60 percent greater throughput than its nearest competitor. The only problem was that no one outside Digital's installed base was buying the drives. In 1994, it decided to sell the technology to a hard-drive manufacturer, Quantum Technology. The rest, as they say, is history. DLT drives then dominated the midrange storage market for quite some time.

9.3.6.13. DLT-S drives (aka Super DLT)

Quantum improved on the original DLT product with what it called the Super DLT. The early Super DLT drives could actually read older DLT media, and this was a great competitive advantage against any other drive, because most companies had a huge pile of DLT tapes with old backups on them. Since the Super DLT actually used a completely different read-write head, Quantum achieved backward compatibility with a separate read head for the older media. Quantum now calls this line DLT-S.

9.3.6.14. DLT-V drives (aka Value DLT)

Some entrepreneurs felt there was still a demand for lower-end DLT drives, so Quantum licensed DLT technology to a company called Benchmark, which began making a value-based line of DLTs. These drives offered smaller capacities and slower throughput rates than Quantum's Super DLT drives, but it did so at a smaller price point as well. The Benchmark line was successful, so Quantum bought Benchmark and now markets these drives as its value DLTs, or DLT-V.

9.3.6.15. DTF drive

This is Sony's entrance into the high-capacity, high-speed market. These drives offer high capacity and aggressive throughputs, and the media is somewhere between the size of an LTO tape and a VCR tape. You typically won't see DTF drives in anything but a Sony tape library. Most libraries support similar-sized media such as 4 mm, 8 mm, or half-inch media, as they can easily retool a library by simply swapping out the drives and tapes. This unfortunately leaves DTF media out in the cold due to its form factor.

9.3.6.16. LMS NCTP drive

Phillips LMS originally made this drive, but Phillips LMS was sold to Plasmon. The Laser Magnetic Storage (LMS) NCTP drive can read or write to 3480 and 3490E cartridges. Although they have not gained wide acceptance with other automation vendors, they have moderate throughput rates and capacities, and Plasmon does offer its own line of tape libraries using these drives. (They have a similar size issue to the DTF media.)

9.3.6.17. LTO drives

Linear Tape Open drives are the product of the LTO Consortium between HP, IBM, and Quantum, and are the most popular midrange tape drives on the market today. Some people seem to like that there are multiple companies that make these drives, which is something that isn't true of the Quantum DLT drives. Competition has done its job, with each member of the consortium working hard to make its version of LTO the best, while making sure its LTO was able to read tapes made by other members of the consortium.

You've got to hand it to Quantum. The LTO Consortium, originally started by HP, IBM, and Seagate, was intended to compete with Quantum's DLT line. Seagate spun off Certance and gave it the LTO line, and then Quantum acquired Certance. Now Quantum is one of the members of the consortium that was designed to take it out. In addition, in 2006, Quantum acquired the only major competitor to its tape library business that wasn't owned by a major OEM. IBM owns its tape library business, and Sun owns what used to be StorageTek. The only remaining major competitor was ADIC, and it is now owned by Quantum.


LTO tapes are half-inch cartridges, and LTO drives offer very fast transfer rates and large capacities at a relatively moderate (although not inexpensive) price point. Most LTO drives also offer some degree of variable speed, and are able to step down to roughly half their original speed in order to keep up with slow incoming data rates.

9.3.6.18. Mammoth drive (end-of-lifed)

The Mammoth drive was Exabyte's attempt to go on its own. It listened to the complaints from its customers about the original 8 mm line but was unwilling to completely reject the format. It believed that the failure of the original line was due to the consumer-grade components produced on the camcorder assembly line. It decided, therefore, to go on its own and produce its own mechanism. It increased the form factor so that the parts wouldn't be so cramped, and it upgraded several key parts of the design. This was a complete redesign, including everything from metal thickness to a different kind of material for the capstan rollers. The Mammoth drives offered much greater capacities and throughput rates than the original 8 mm drives, and Exabyte sold quite a few for a while. However, it eventually pulled the drive from the market due to lack of demand.

9.3.6.19. MLR 1-3 drives

Just as the DLT drive was a modified TK 70, the MLR drives are a completely new drive based on QIC media. MLR drives offer moderate capacities and speeds but have not gained wide-spread acceptance in the data center or from automation vendors.

9.3.6.20. VXA

Originally developed by Ecrix, the VXA technology was acquired by Exabyte. VXA cartridges offer an option to those looking for an inexpensive tape drive with reasonable capacity and performance. VXA makes many very strong claims about the reliability of data written to VXA tapes, including testimonies from people who were still able to read their tapes after very bad things happen to those tapes. Although Exabyte offers some tape libraries using VXA, VXA has not been adopted by major automation vendors. Again, it is probably the difference in the form factor.




Backup & Recovery
Backup & Recovery: Inexpensive Backup Solutions for Open Systems
ISBN: 0596102461
EAN: 2147483647
Year: 2006
Pages: 237

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