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Storage systems are built by taking the basic capability of a storage device, such as the hard disk drive, and adding layers of hardware and software to obtain a highly reliable, high-performance, and easily managed system. We explain in this paper how storage systems have evolved over five decades to meet changing customer needs. First, we briefly trace the development of the control unit, RAID (redundant array of independent disks) technologies, copy services, and basic storage management technologies. Then, we describe how the emergence of low-cost local area data networking has allowed the development of network-attached storage (NAS) and storage area network (SAN) technologies, and we explain how block virtualization and SAN file systems are necessary to fully reap the benefits of these technologies. We also discuss how the recent trend in storage systems toward managing complexity, easeof-use, and lowering the total cost of ownership has led to the development of autonomic storage. We conclude with our assessment of the current state-of-the-art by presenting a set of challenges driving research and development efforts in storage systems.The first data storage device was introduced by IBM in 1956. Since then there has been remarkable progress in hard disk drive (HDD) technology, and this has provided the fertile ground on which the entire industry of storage systems has been built. Storage systems are built by taking the raw storage capability of a storage device such as the HDD and by adding layers of hardware and software in order to obtain a system that is highly reliable, has high performance, and is easily manageable. Storage systems are sometimes referred to as storage subsystems or storage devices (although device is better used to describe the raw storage component or an elementary storage system). Originally, the storage system was just the HDD, but over time storage systems have developed to include advanced technologies that add considerable value to the HDD. Storage systems have evolved to support a variety of added services, as well as connectivity and interface alternatives. It is for this reason that file systems and storage management systems are often considered parts of a storage system and thus will be briefly treated in this paper.To understand the evolution of storage systems, it is important to observe the evolution of the HDD. The areal density of the HDD has improved by seven orders of magnitude, and this has resulted in a reduction of the floor space taken by the corresponding storage systems also by about seven orders of magnitude. Figure 1 plots the HDD areal density (on the left) and the price of various storage devices (on the right) since 1980. HDD prices have decreased by about five orders of magnitude since 1980, while the cost of storage systems has fallen about 2.5 orders

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The BXI, works like a TtX if the SEcond operand (in this ease 3) is odd; the BXH with an even second ~vrgument allows the increment and comparand to be in adjacent regist.ers, in this ease 4 aud 5. The Stl. subtraets register 5 from register 2. C is a fuI1 word comparison which sets the condition codE; this example could be modified for a halfword ~able by changing (3 to CH and "4" to "2" throughout the instructions above. BC branches (>~ eondition code 6, which is 0110 in binary--.-i.e., first opera.nd low or first operand high.The timi~g iu this ~.xa.mpl< depend:, ~>~ d~<, ,~o~i(1. (3,~ the ),L>d ;~0, th~ (imiugs in microseeouds of the givm~ instructions arE: BXI,, ;;S; I;XH, 3SiS l{, 30:('. SO: BC, 15. ThE total is 151 nficrose('~u~d, \ si~!{]b' muii it@ i> struetion on the Mod ;gt) t~kes 2;~5 mi,.:roa'c(mds. (h~ ~ther models the situ0, iiOlt iS R)SS clear, bl, l[ iD, ttlt} (VeIlb ;~i legist five instructions ~re needed: multiply, ma.sk, BXtI ~0 TFUIA. C, and B(L Note that ~,> this machine ~ cam~ot "enclose" a BXH ~ithi~ :~ comparison tweau,;c ~h~: C instruetii:m does not skip, but merely >e{s thv co~dhiot~ code. Scatter Storage TechniquesP~ OBERT 3;[ORRIS Be8 Te/epke~e Laboratories, 3iurra9 .Hill, N. Y. Scatter storage techniques as a method for implementing the symbol tables of assemblers ~and compilers are reviewed and a number of ways of using them more effectively ore presented. Many of the most useftA variants of ~e techniques are documented. KEY WORDS AND PHRASES: *co~r storage, ho~ addressing, searching/, file scorching, file oddre~i~g, s~or~ge loyout CR CATEGORIES: 3.73, 3.74