Published: 03 Jul 2002
Tape has always been the media of choice for file backup and archiving tasks. Its low cost, portability and simplicity of use make it a perennial favorite with storage administrators. Ongoing improvements in tape drive and media performance, capacity and overall reliability have helped tape maintain its dominating position as the medium of choice for most backup operations. Today, whether employed as an offline safe backup, or online archival storage in automated libraries, tape is an intricate part of any overall hierarchical storage management plan.
Following is a brief overview of the most popular tape choices for midrange systems and networking segments, and their technologies, performance parameters and costs. We'll then put on rose-colored glasses and peer into tape's future.
Major 8mm formats
8mm tape technology employs helical scan recording methodology and is a reliable method for storing large volumes of computer data. With this approach, data is written in short diagonal tracks across the width of the tape. This is the same type of recording technique used by videotape recorders. The full-tape (beginning-to-end) is written in one pass. Basically, helical scan devices utilize a tape transport system consisting of a two-reel cartridge and a tape path that pulls tape over (and halfway around) a spinning read/write cylinder. Helical scan systems can achieve extremely high data densities and have proven field-reliable since they began wide-scale deployment in 1996.
Advanced Intelligent Tape (AIT). Developed by Sony, it's designed to support the upper-end midrange server market with a combination of data integrity, speed and capacity. AIT tape drive systems are primarily designed for use in tape libraries and robotic devices used in backup applications. AIT is known for its performance, media and Memory-In-Cassette (MIC) features.
Sony's new AIT-3 8mm tape product features fast (27 seconds) access, high-density tape recording (260GB, compressed) and reliable (50,000-hour head life; 30,000 tape passes) backup for large-scale network servers with large volumes of images and data to be protected. Sony AIT-3 drives provide a 31MB/s (compressed) data transfer rate at a media cost of approximately $0.38/GB.
AIT's Advanced Metal Evaporated (AME) media is a data-grade tape, with anti-corrosive properties that improve tape durability and reduce tape wear, extending media life to a 30-year archival rating. An AIT-3 media cartridge costs about $99/cartridge. Sony AIT drives are available in a compact 3.5-inch footprint (with standard frame mount optional). Interfaces supported include all versions of SCSI.
The MIC hardware consists of a 64Kb (AIT-3 models) erasable electrical programmable read-only-memory chip mounted inside the data cartridge. The MIC stores the tape system log, user-defined information and all of the file storage location data normally found on the first segments of tapes used in competing technologies. Because the MIC firmware gauges how far to fast forward or rewind, the drive electronics no longer must read individual address ID markers while the tape is moving. As the target zone approaches, the drive motor slows down to pick up the physical ID markers for fine positioning. The result is search speeds up to 150 times the normal read/write speed of the drive, resulting in lower average data access times. The MIC feature also provides an additional level of reliability, due to the duplication of file index search information.
Mammoth tape. This is Exabyte's 8mm entry into the high-end midrange server market, featuring a tape deck with 40% fewer parts than previous 8mm drives, a capstan-less design that improves reliability by reducing tape wear and support for AME media. Mammoth drives also feature self-calibration and automatic device monitoring for preventive maintenance and error recovery, accompanied by associated automatic reports.
A major benefit of Mammoth tape systems is its backward compatibility with previous Exabyte standard 8mm drives, libraries and media. Users can upgrade while protecting their investment in current equipment. Mammoth-2 8mm tape systems provide an average 60-second file access, high-density tape recording (up to 150GB, compressed) and reliable (50,000-hour head life; 20,000 tape passes) backup for large-scale network servers requiring storage of large volumes of images and data. Mammoth drives provide a 30MB/s (compressed) data transfer rate at a media cost of approximately $92/cartridge or $0.61/GB.
Mammoth supports the same Advanced Metal Evaporated (AME) media as AIT, and, therefore, has the identical archival rating. Exabyte's drives come in a 5.25-inch, half-height footprint and offer a Fibre Channel (FC) interface as well as support for all versions of SCSI. This technology is compatible with Unix, Linux and NT environments.
What's in store for 8mm? AIT-3, which began shipping last January, is the first of four planned generations (AIT-4, AIT-5 and AIT-6). Sony is committed to doubling capacity and transfer rates every 18 months, with the AIT-6 reaching 800GB (native), 96MB/s (native).
Sony is also working on a new family of drives (S-AIT) slated for introduction at the end of this year. The first generation will have a 500GB capacity (native) with a 30MB/s (native) throughput rate. The S-AIT road map calls for four generations, reaching a native capacity of 4TB.
Mammoth-3 from Exabyte is ready to begin shipping. It has a 120GB capacity (native) and a transfer rate of 14MB/s (native). A new super drive that combines Mammoth and VXA tape technology is also under development and is expected to be released in an 18-month time frame. Capacities and transfer rates have not yet been finalized.
Digital Linear Tape
Digital Linear Tape (DLT) technology has been a leader in high-end tape backup for many years. These tape devices are suited for situations where the backup window is small and the amount of data to be backed up continues to grow. The new Super DLT (SDLT) tape targets the high-end midrange systems, network servers and mainframe backup and archiving markets. DLT tape technology has been around since the 1980s and has proven to be rugged and dependable in full-duty cycle environments. It's easy to integrate, reliable enough for the most demanding applications and SDLT is backward compatible with previous DLT generations.
DLT technology is a modern adaptation of the old reel-to-reel tape methodology where the tape cartridge performs as the source reel and the tape drive as the take-up reel. The tape is guided, not pulled, past a stationary head, in contrast to helical scan's spinning head and pulling technique. The recorded side of the tape never touches the guides, minimizing tape wear. The drive utilizes 1/2" tape (this is 60% wider than 8mm tape) and records by writing data in a serpentine pattern on parallel tracks. Each track spans the entire length of the tape. When the end of the tape is reached, the heads are repositioned to record a new set of tracks and recording again continues over the entire length, this time in the opposite direction.
The current generation of Super DLT drives, (SDLT 220), provides 70 second file access, storage capacity of 220GB (compressed) with an estimated shelf life of 30 years and a 30,000 hour head life, with a rating of 1,000,000 tape passes. SuperDLT drives are capable of data transfer rates up to 22MB/s (compressed).
However, having a fast tape drive doesn't ensure the highest throughput. If the transfer rate of a tape drive is faster than the host data rate, the tape must stop and reposition frequently, degrading performance. DLT technology incorporates a feature called adaptive cache buffering to achieve maximum throughput. The drive monitors the host system and dynamically adjusts the cache buffering operations to match the host data rate, thus minimizing delays due to repositioning.
What's in store for DLT? Quantum's latest road map starts with the SDLT 320 expected out later this year. There will be four generations of SDLT drives, with a new product introduction every 18 months. Each generation will double capacity and transfer rates until topping out at 1.2TB (native), 100+ MB/s (native).
Linear Tape Open
Linear Tape Open (LTO), developed by a consortium of IBM, Seagate, and Hewlett-Packard, is an attempt to break the proprietary hold Quantum has on high-capacity linear tape and to offer an open standard alternative. LTO formats employ a multichannel linear serpentine recording method similar to DLT (data is recorded in tracks that run down the length of the tape; the tape makes multiple passes past the read/write heads from one end to the other and then reverses direction).
LTO Accelis utilizes a two-reel, self-contained 8mm cartridge. The cartridge loads with equal amounts of tape on each reel, minimizing access times. Accelis will serve automated environments needing tape storage solutions for a wide range of online data inquiry and read-intensive applications.
LTO Ultrium cartridges are of a single-reel design and utilize half-inch tape media. Tape is extracted from the cartridge by the drive and wound onto a take-up reel contained within the drive itself. Ultrium is suited for backup, restore and archive applications in either standalone or automated environments. LTO cartridges include a built-in cartridge memory. This is an RF module with a non-volatile memory (4Kb); it provides a duplicate file log of cartridge data positioning and user specified information. An external reader allows immediate access to that information without having to insert the cartridge into a drive. A typical Ultrium cartridge costs approximately $137.
What's in store for LTO? IBM, Seagate, and Hewlett-Packard are committed to a four-generation LTO road map and a doubling of capacity and transfer rates with each new product.
In the fourth generation, Accelis capacity will be boosted to 200GB (native) with data transfer rates ranging between 80MB/s to 160MB/s (native). Access times are expected to be less than seven seconds. Ultrium capacities will increase to 800GB (native) with data transfer rates ranging between 80MB/s to 160MB/s (native).
Optical tape: The new kid on the block
- A new array write head architecture which yields a 32-channel head, facilitating high sustained data recording rates and high data storage density.
- An optical head system architecture providing simultaneous readback of the 32 channels without channel alignment issues; this provides for high sustained data transfer rates.
- A new cartridge design which incorporates extra wide short tape, reducing data access times.
What's in store for optical tape? O-Mass plans to introduce a new drive every 18 months spanning four generations, beginning with its initial product shipments. Each generation will double capacity and transfer rates resulting in a fourth generation capacity of 10TB (native) and a 1GB/s (native) transfer rate.
Knowing what tape technology to use is your job, but sometimes it's not always easy to cut through the marketing hype to make smart comparisons (see "How tape formats compare" below).
The 8mm market is dominated by AIT, which offers an excellent price performance offering for departmental backup and recovery applications. It also provides high density in tape library applications because of its small media size. Mammoth technology is good in these same areas, but the company was late to market, which enabled AIT to take the lead.
DLT technology works well in large tape library applications and in data backup applications, but isn't the best choice for applications such as HSM that require fast access to the first data block.
LTO technology offers the best combination of heavy-duty use and fast access to data at a reasonable price. LTO Accelis will be used primarily for single drive applications at the entry level.
Optical-assisted tape holds the promise of high capacities on a very robust media. However, with the price of disk storage dropping rapidly, it will have to be low enough to be an attractive alternative to existing options.