Tape It Simple

Today's tape storage marketplace is

made up of a confus-ing array of formats and technologies that overly complicate customer

buying decisions. OEMs and analysts agree that the situation is not serving customers well

and have called for open formats.






Open formats benefit customers by offering them multiple sources of product and media, and
by providing a means to enable compatibility between products from different vendors. The

competitive environment fostered by multiple sources of product also leads to faster paced

innovation and enhancements, and provides for data compatibility and investment protection

well into the future. For OEMs, it shortens the qualification cycle and reduces

complexities in system configuration and product planning.






Hewlett-Packard, IBM and Seagate have responded to this need for open formats by
collectively developing new open format specifications based on the group's Linear

Tape-Open (LTO) technology. The group developed the formats to serve multiple market areas

and to be supported by multiple suppliers.






Supporting a range of needs


To best serve customers across the entire spectrum of applications, the three core
technology providers designed LTO technology to accommodate a range of storage

requirements from single server to complex network environments, in both fast-access and

high-capacity formats. For this reason, two implementations of the technology were

developed. Ultrium and Accelis. Each is designed to meet customer needs well into the next

decade.






The Ultrium tape format is the implementation of LTO technology optimized for high
capacity and performance with outstanding reliability, in either stand alone or automated

environment. The Ultrium tape format uses a single reel cartridge to maximize capacity. It

is ideally suited for backup, restore, and archive applications. The Ultrium tape format

will establish a new benchmark for large volume backup and archive.






The Accelis tape format is the implementation of LTO technology optimized for fast access
to data, with exceptional reliability and performance characteristics. It uses a two-reel

cartridge that loads at the middle of the tape to minimize access time. The Accelis tape

format is targeted at automated environments and can enable a wide range of

"on-line" data inquiry and retrieval applications.



Although there are differences between Ultrium and Accelis, developers and customers will
benefit from the sharing of key technology characteristics of LTO through economies of

scale for certain components, which should result in an overall cost benefit.






Essentially a linear serpentine technology, LTO is the best available implementation of
this technology in many ways. First, it enables a higher number of concurrent channels.

The first generation of LTO enables up to eight channels, with future versions delivering

up to 16 channels. By comparison, most linear serpentine implementations offer a maximum

of four channels-the notable exception being the Magstar format from IBM.






LTO was also developed to take advantage of the best available servo and head
technologies, which are essential to provide the high degree of accuracy critical for high

track densities. The more tracks there are on a tape, the greater is the risk of

overwriting a neighboring track. LTO's track following servo monitors and controls the

position of the head on the tape to ensure accuracy and avoid overwrites.






In addition, LTO will offer a higher recording density (100 Mbits/sq inch) than any linear
tape technology available in the industry today. The high linear density is supported by a

robust logical format including lock step recording across multiple data tracks of user

data protected by a true cross-product Error Correction Code (ECC) designed for robust

multiple track operation even if random error rates become high.






Multi-channel design


The innovative LTO design is based in part on a bi-directional, multi-channel adaptation
of linear technology already in use today. The LTO format specification divides the full

tape width into smaller data bands to minimize the effects of tape shrinkage over time.

The number of these data bands depends on implementation. Ultrium has four data bands and

Accelis has two. The tape head spans one band, and fills each data band sequentially. At

the top and bottom of each data band is a servo band. The servo bands provide location

information to the head as it writes and verifies data tracks within that band. The

process of positioning the head on the tape is a dynamic interaction between the head, the

media and the servo elements of the system. The innermost bands are written first. To

afford protection to data written in the outermost bands, two blank areas called edge

guard bands border the top and bottom edges of the tape.






Tape head movement is controlled by the servo system, using information encoded in servo
bands bordering both sides of each data band. Two types of information are encoded into

the servo bands. The first provides a cross-tape position error signal using the robust

Timing Based Servo (TBS). The second provides absolute down-the-tape (longitudinal)

positioning information, which allows the tape to span and maneuver along the length of

the track for precise data handling operations. The absolute location down the length of

the tape is recorded in longitudinal position (LPOS) marks information encoded into the

TBS servo code.






The format specifies multiple servo positions within each servo band. The number of servo
positions depends on the implementation (six for Ultrium, eight for Accelis). The distance

between two adjacent servo positions corresponds to the distance between the data tracks.






LTO's robust logical format permits a higher recording density than any linear tape
technology available in the industry. The cross-product ECC is interleaved both across

tracks and along tracks over a large block size. This ECC is powerful enough to ensure

reliable recovery of data even with the loss of one of eight tracks on a read operation

and to 1% of the bytes on the remaining tracks being in error.






Dynamic rewrite of bad blocks permit writing through a region of tape which is bad in some
tracks. It is also powerful enough to permit recording a good copy of all user data even

if one or more write heads is nonfunctional. Beyond rewrite of bad data blocks, data is

protected by dynamically discontinuing write operations in any region of tape where the

servo's position error signal is unreliable. The data is then rewritten down tape where

the servo signals are again reliable.






Finally, the LTO-CM is an electronics and interface module, embedded in both the Ultrium
and Accelis cartridges, which contains 4 Kb (32 Kbits) nonvolatile memory for storage and

retrieval of information about the cartridge and data. LTO-CM uses a non-contacting radio

frequency (RF) interface that eliminates the need for a physical power or signal

connection inside the cartridge. LTO-CM permits the drive to know where on tape a record

exists without requiring reading of any directory or table off the tape. LTO-CM also

permits blocks to be recorded with an absolute Write Pass number and sequence number.





Ultrium: High capacity


The technology providers developed Ultrium to enable high-capacity, high-performance tape

back-up solutions for the data storage industry. Optimized for high capacity and transfer
rates, Ultrium provides a technologically solid migration path that allows for maximum
expandability. The first generation of this technology allows for storage of up to 100GB

of data (>200GB compressed) on a single cartridge at data rates as high as 20MG/second

(>40MB/second compressed). The performance is made possible through the use of linear

recording techniques that employ either a four or eight channel head and closed loop servo

technologies. The high capacity results from applying LTO to a new data cartridge that has

been designed to maximize the amount of tape surface area while still enabling very small

form factors. The first generation cartridge will contain 600 meters of cartridge memory

chip that is part of the LTO interchange specification. The drive mechanism will provide

bi-directional tape motion during read/write and locate/rewind operations. It will also

include the read/write head, a mechanism to control the positioning of the head, sensors

to detect the presence and write-protect status of the cartridge and the LTO-CM

electronics and interface.






Ultrium's four-or
eight-element array of read and write elements, and the extra servo read elements in the

head may be interleaved, merged, or arranged in modules. The elements are arranged on the

head in pairs such that a data pattern written by a write head is verified immediately by

a paired read head. The heads must be capable of reading and writing while the tape is

moving in the forward or the reverse direction.






Ultrium partitions the half-inch tape into 384 data tracks, evenly divided into four data
bands. The number of tracks in a data band is the same whether the data is written by a

four-or eight-element head. If an eight-channel head is used, the 96-track data band

results from six head positions dictated by the servo. The four-channel implementation

requires extra servo readers on the head to generate 12 head positions from the six

pre-defined positions.






The Ultrium single-reel cartridge design uses a take-up reel that is located inside the
drive. The tape is engaged by means of a coupler that "grabs" a leader pin at

the start of the tape and guides it around the tape head to the take-up reel in the drive.

After the leader pin is secured in the take-up reel, the reel rotates and pulls the tape

through the tape path. The drive's reel motor is coupled to the cartridge by means of a

clutch with a toothed rim.






Accelis: Faster tape


The Accelis format will utilize all the advantages of LTO described for Ultrium except
that the capacity has been reduced for the purpose of improving the access time to data

dramatically. The capacity reduction is the result of making the tape narrower (hence

fewer tracks) and by putting two reels in the cartridge instead of one. This results in

216 meters of 8mm wide tape, which contains 25GB of data (>50GB compressed). The dual

reel cartridge implementation provides a number of advantages. The tape does not have to

be threaded around to take up reel at all, enhancing the reliability of Accelis, which is

particularly critical in a library environment. Because the tape path is fully contained

in the cartridge, the tape can be loaded at mid-point which, in conjunction with the

eliminated thread time, improves search time to sub-10 second averages. The Accelis

implementation of LTO is an ideal choice for automated tape solutions focusing on digital

library applications and other read-intensive tape applications.



Accelis records the LTO format in two data bands instead of four for Ultrium and each data
band contains 128 tracks (256 tracks total). Like Ultrium, the data bands can be recorded

with eight-channel or four-channel heads and eight servo positions are defined in the

servo band to support the recording.






The unique
aspect of Accelis cartridge design is that the media could be pre-positioned or

post-rewound to further improve system performance. Also, in conjunction with the LTO-CM

an offline device, such as a smart picker or a queing device, can locate to a specific

point on tape prior to tape drive insertion. This is a very powerful concept that will

enable unprecedented solutions unique to Accelis.






Courtesy: Hewlett-Packard, IBM and Seagate.