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212 CHAPTER 6 / EXTERNAL MEMORY

Table 6.6 LTO Tape Drives

LTO-1 LTO-2 LTO-3 LTO-4 LTO-5 LTO-6
Release date 2000 2003 2005 2007 TBA TBA
Compressed 200 GB 400 GB 800 GB 1600 GB 3.2 TB 6.4 TB
capacity
Compressed
transfer rate 40 80 160 240 360 540
(MB/s)
Linear density
(bits/mm) 4880 7398 9638 13300
Tape tracks 384 512 704 896
Tape length 609 m 609 m 680 m 820 m
Tape width (cm) 1.27 1.27 1.27 1.27
Write elements 8 8 16 16

read-write head is capable of reading and writing a number of adjacent tracks
simultaneously (typically two to eight tracks). Data are still recorded serially along
individual tracks, but blocks in sequence are stored on adjacent tracks, as suggested
by Figure 6.14b.
A tape drive is a sequential-access device. If the tape head is positioned at
record 1,then to read record N, it is necessary to read physical records 1 through N - 1,
one at a time. If the head is currently positioned beyond the desired record, it is nec-
essary to rewind the tape a certain distance and begin reading forward. Unlike the
disk, the tape is in motion only during a read or write operation.
In contrast to the tape, the disk drive is referred to as a direct-access device.A
disk drive need not read all the sectors on a disk sequentially to get to the desired
one. It must only wait for the intervening sectors within one track and can make suc-
cessive accesses to any track.
Magnetic tape was the first kind of secondary memory. It is still widely used as
the lowest-cost, slowest-speed member of the memory hierarchy.
The dominant tape technology today is a cartridge system known as linear
tape-open (LTO). LTO was developed in the late 1990s as an open-source alterna-
tive to the various proprietary systems on the market. Table 6.6 shows parameters
for the various LTO generations. See Appendix J for details.

6.5 RECOMMENDED READING AND WEB SITES

[JACO08] provides solid coverage of magnetic disks. [MEE96a] provides a good survey of the
underlying recording technology of disk and tape systems. [MEE96b] focuses on the data
storage techniques for disk and tape systems. [COME00] is a short but instructive article on