LTO & SDLT: built SAN tough.
However, in this new era of SAN-based backup, automated tape libraries and the underlying tape drive technology are facing the challenge of dramatically higher duty cycles and rigid uptime and availability requirements. The standard enterprise SAN is now a 24x7x365 environment. Gone are the days when a tape drive was designed around requirements for 10% duty cycles. SDLT and LTO Ultrium are the first tape-drive technologies developed specifically for the high-duty cycle requirements of enterprise-class automated environments, and particularly SANs. The more robust designs of current generation SDLT and LTO drives and media are designed for the rigors of SAN environments, where a single library is utilized to back up data from multiple application servers hosted on SAN-based disk storage.
From the actual recording media to a variety of other drive- and cartridge-based enhancements, SDLT and LTO Ultrium each provide a durable and reliable tape technology platform to handle the demands of SAN tape backup.
Both Quantum's SDLT 320 drives and LTO Ultrium 2 drives--like HP's StorageWorks Ultrium 460--are rated at 250,000 hours MTBF at a 100% duty cycle. The rectal particle LTO Ultrium 2 and Advanced Metal Powder-based SDLT media carry durability ratings of 1,000,000 head passes with an archival life expectancy of more than 30 years.
Improving drive and media reliability and durability is just part of the overall platform improvements that make SDLT and LTO Ultrium so well positioned to handle the demands of SAN-based backup and archiving. Just as critical are the technology enhancements incorporated in these tape formats that enable dramatically higher capacity and transfer rate performance required to handle the data loads for enterprise SAN operation. The improvements were made to virtually every key area of the drives and media: new media and head designs to support higher bit densities and throughput, advanced servo designs to enable higher track densities and more efficient data encoding.
SDLT and LTO Ultrium 2 media both incorporate new cartridge shells designed to withstand high-volume use in robotic libraries. The SDLT cartridge shell has been significantly upgraded compared to the DLT tape IV cartridge with internal support structure and new external materials designed for improved longevity. The buckling system on the SDLT tape uses a new Positive Linking Tape leader, enhancing the performance reliability of the tape in automated environments, and ensuring more accurate tape loading.
The tape inside the SDLT cartridge leverages multiple technical advances over previous DLT generations to enable higher capacity as well as improved media durability and recording reliability. To reach the desired capacity targets for enterprise SAN data loads, SDLT media uses a new 1900-oersted Advanced Metal Powder (AMP) magnetic coating capable of supporting the SDLT roadmap to 1TB per cartridge. The AMP media produces metal particulates with a much finer grain size, producing a recording medium able to take full advantage of the high recording density potential of the magneto resistive head clusters used in these drives. The fine grain particles are complemented with a new binder composition that enables an extremely thin magnetic layer, allowing shorter wavelength and higher bit densities and data rates. SDLT media also incorporates a new lubricant that supports the higher duty cycles demand by reducing friction between the tape and the read/write heads, leading to extended head and tape life.
LTO Ultrium 2 media uses the same physical cartridge as Ultrium 1 but, like SDLT, has moved to a higher coercivity 1850-oersted metal particle media formulation to support the higher recording density and capacity requirements. The Ultrium 2 media retains the LTO-Cartridge Memory feature, a 4KB memory chip combined with a radio frequency (RF) interface that stores cartridge-specific information about the data and cartridge usage. LTO-CM permits the drive to know where on tape a record exists without reading a directory or table off the tape and without the tape being loaded into a drive. The Cartridge Tape Module feature of SDLT media provides the same type of file directory and tape usage information by writing that data directly to the tape.
Staying on Track
Both SDLT and LTO incorporate servo positioning systems to ensure accurate head position when using very high track densities. In the case of SDLT 320, there are 448 serpentine data tracks across the half-inch tape, compared to 512 tracks with the LTO Ultrium 2 format. To support these high track densities, both formats have embraced servo-positioning technology, but their methods are quite different.
The LTO 2 format utilizes five pre-written, non-erasable servo bands bordering four data bands. Each data band consists of 128 data tracks, 64 tracks in each direction. Servo bands on each side of the current data band use the signal read by the servo MR heads to precisely position the MR read/write heads on the desired data track. The active servo elements are always arranged to be on the opposite head 'bump' to the read/write elements, preventing interference from the write head elements.
The SDLT 320 format uses a radically different approach to servo tracking. Quantum calls the SDLT recording/servo system Laser Guided Magnetic Recording technology. SDLT media reserves the entire front side of the tape for data tracks, while three optical servo tracks are pro-written on the back of the tape during the manufacturing process. In this design, the servo positioning system combines both optical and magnetic elements with a pivoting mechanism that protects the head and actuator from vibrations and lateral tape movement, delivering more accuracy and enabling higher track densities. The LGMR technology also allows for future increases in track densities by completely removing the servo tracks from the data side of the tape.
A Move to MR Heads
LTO and SDLT formats utilize magneto resistive heads that are able to support higher densities and data rates, while minimizing the negative impacts of temperature and humidity variations.
The SDLT design incorporates the Magneto Resistive Cluster (MRC) head design into LGMR assembly along with the Pivoting Optical Servo. MRC heads are joined together through the use of advanced thin-film processing technology to create a densely packed head cluster. The SDLT format uses an 8-MRC head stack to read or write 8 channels of data with each tape pass. The MRC provides higher data transfer rates and capacity than traditional MR heads of equal size. MRC heads also provide high wafer usage efficiency for low head costs.
The MR heads used in LTO Ultrium 2 drives are slightly modified versions of Ultrium 1 heads, with tighter head gaps for higher density recording. In order to achieve higher track density, the Ultrium 2 format uses a "track trimming" technique that overlaps adjacent tracks in order reduce the track width. The heads are a two-bump, self-cleaning design with magneto-resistive (MR) read heads and inductive write heads. The heads are also designed to write and read Ultrium 1 format tapes without the need for a second head stack to provide backwards compatibility.
PRML: Higher Efficiency Data Coding The extremely high-bit densities of second-generation super drives (a linear density of 193,000 bpi in the case of SDLT 320 and 7.32Kbs per millimeter bpi for LTO Ultrium 2) required capabilities beyond the conventional Run Length Limited 1,7 read channel technology with peak detection common to many tape drive formats. To solve this problem, both formats have adopted high-efficiency Partial Response Maximum Likelihood (PRML) technology, a well-proven disk- drive technique to enable higher recording densities that has recently started to appear in formats.
Peak detection focuses on recognizing peak voltage levels for interpreting data from the head. But as bits are packed more densely on the tape, it becomes harder to distinguish data from background noise and other interference to detect separate peaks for individual bits. PRML technology solves this problem by first converting the analog read signal to a digital signal, then using the digital signal as the basis to recognize data bits. This process enables PRML technology to resolve higher bit densities than peak detection, while also improving noise rejection, in effect producing higher efficiency data encoding. For example, the SDLT PRML implementation achieves 97% efficiency, meaning that for every 100 physical flux changes written, the PRML create 97 data bits. RLL encoding typically achieves about 67% efficiency.
Both SDLT220 and 320 drives are based on PRML technology, while it is a new feature on LTO Ultrium 2 drives; Ultrium 1 drives use RLL 1,7. However, the LTO PRML implementation can also recover Ultrium 1 format signals. Ultrium 2 drives identify Ultrium 1 media from the cartridge memory system and then adjusts the data decoding and digital filtering to handle RLL 1,7 signals.
Attaching to the SAN
SDLT 320 drives are sold with Ultra SCSI connections, while LTO Ultrium 2 drives are available in both Ultra SCSI and native Fibre Channel (FC) versions. The lack of native FC support does not impact the ability of SDLT drives to function in SAN environments, as these super drives are typically installed in an automated library which is interfaced to the FC SAN, not the individual drives. The major independent tape library suppliers, including ADIC, Quantum/ATL and StorageTek, all offer FC SAN connectivity options for their mid-range and enterprise-class libraries, allowing customers to easily take advantage of the SAN-capable performance, capability and reliability of SDLT and LTO Ultrium 2 drives in their SAN infrastructures. Older libraries that are being upgraded with LTO and SDLT drives but lack direct FC support can still be deployed on SANs via SAN gateways that provide SCSI-Fibre Channel bridge functionality, opening legacy libraries to the advantages of SAN-based backup and tape management.
Storage Area Networks have transformed enterprise storage, including placing stringent new reliability, capacity and performance demands on tape backup technology. The tape industry has responded to this challenge with two equally robust platforms designed with SAN-class requirements in mind. SDLT and LTO 2 Ultrium not only boast the capabilities to support the current enterprise SAN requirements, but have the technology foundations in place to reach one terabyte per cartridge and beyond.
Peter N. Brinkman is vice president of marketing at Maxell Corporation of America (Fair Lawn, NJ)
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|Author:||Brinkman, Peter N.|
|Publication:||Computer Technology Review|
|Date:||Oct 1, 2003|
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