Advances in Data Storage Technology Contents I. Introduction3 II. Purpose of storage4 III. Hierarchy of storage6 A. Primary storage6 B. Secondary storage7 C. Tertiary storage7 D. Off-line storage8 IV. Characteristics of storage9 A. Volatility9 B. Mutability9 C. Accessibility10 D. Addressability10 E. Capacity11 F. Performance11 G. Energy use11 V. Fundamental storage technologies12 A. Semiconductor12 B. Magnetic12 C. Optical13 D. Paper14 E. Uncommon14 VI. Related technologies17 A. Network connectivity17 B. Robotic storage17 References19 I. INTRODUCTION
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data used for computing for some interval of time. Computer data storage provides one of the core functions of the modern computer, that of information retention. It is one of the fundamental components of all modern computers, and coupled with a central processing unit (CPU, a processor), implements the basic computer model used since the 1940s. In contemporary usage, memory usually refers to a form of semiconductor storage known as random-access memory (RAM) and sometimes other forms of fast but temporary storage.
Similarly, storage today more commonly refers to mass storage — optical discs, forms of magnetic storage like hard disk drives, and other types slower than RAM, but of a more permanent nature. Historically, memory and storage were respectively called main memory and secondary storage (or auxiliary storage). Auxiliary storage (or auxiliary memory units) was also used to represent memory which was not directly accessible by the CPU (secondary or tertiary storage). The terms internal memory and external memory are also used. II. Purpose of storage
Many different forms of storage, based on various natural phenomena, have been invented. So far, no practical universal storage medium exists, and all forms of storage have some drawbacks. Therefore a computer system usually contains several kinds of storage, each with an individual purpose. A digital computer represents data using the binary numeral system. Text, numbers, pictures, audio, and nearly any other form of information can be converted into a string of bits, or binary digits, each of which has a value of 1 or 0. The most common unit of storage is the byte, equal to 8 bits.
A piece of information can be handled by any computer whose storage space is large enough to accommodate the binary representation of the piece of information, or simply data. For example, using eight million bits, or about one megabyte, a typical computer could store a short novel. Traditionally the most important part of every computer is the central processing unit (CPU, or simply a processor), because it actually operates on data, performs any calculations, and controls all the other components. Without a significant amount of memory, a computer would merely be able to perform fixed operations and immediately output the result.
It would have to be reconfigured to change its behavior. This is acceptable for devices such as desk calculators or simple digital signal processors. Von Neumann machines differ in that they have a memory in which they store their operating instructions and data. Such computers are more versatile in that they do not need to have their hardware reconfigured for each new program, but can simply be reprogrammed with new in-memory instructions; they also tend to be simpler to design, in that a relatively simple processor may keep state between successive computations to build up complex procedural results.
Most modern computers are von Neumann machines. In practice, almost all computers use a variety of memory types, organized in a storage hierarchy around the CPU, as a trade-off between performance and cost. Generally, the lower a storage is in the hierarchy, the lesser its bandwidth and the greater its access latency is from the CPU. This traditional division of storage to primary, secondary, tertiary and off-line storage is also guided by cost per bit. III. Hierarchy of storage A. Primary storage: Primary storage (or main memory or internal memory), often referred to simply as memory, is the only one directly accessible to the CPU.
The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner. Historically, early computers used delay lines, Williams’s tubes, or rotating magnetic drums as primary storage. By 1954, those unreliable methods were mostly replaced by magnetic core memory. Core memory remained dominant until the 1970s, when advances in integrated circuit technology allowed semiconductor memory to become economically competitive. This led to modern random-access memory (RAM).
It is small-sized, light, but quite expensive at the same time. (The particular types of RAM used for primary storage are also volatile, i. e. they lose the information when not powered). As the RAM types used for primary storage are volatile (cleared at start up), a computer containing only such storage would not have a source to read instructions from, in order to start the computer. Hence, non-volatile primary storage containing a small startup program (BIOS) is used to bootstrap the computer, that is, to read a larger program from non-volatile secondary storage to RAM and start to execute it.
A non-volatile technology used for this purpose is called ROM (Read-only memory). Recently, primary storage and secondary storage in some uses refer to what was historically called, respectively, secondary storage and tertiary storage. B. Secondary storage: Secondary storage (or external memory) differs from primary storage in that it is not directly accessible by the CPU. The computer usually uses its input/output channels to access secondary storage and transfers the desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down—it is non-volatile.
Consequently, modern computer systems typically have two orders of magnitude more secondary storage than primary storage and data is kept for a longer time there. In modern computers, hard disk drives are usually used as secondary storage. Rotating optical storage devices, such as CD and DVD drives, have longer access times. Some other examples of secondary storage technologies are: flash memory (e. g. USB flash drives or keys), floppy disks, magnetic tape, paper tape, punched cards, standalone RAM disks, and Iomega Zip drives. C. Tertiary storage: Tertiary storage or tertiary memory provides a third level of storage.
Typically it involves a robotic mechanism which will mount (insert) and dismount removable mass storage media into storage device according to the system’s demands; this data is often copied to secondary storage before use. It is primarily used for archival of rarely accessed information since it is much slower than secondary storage (e. g. 5–60 seconds vs. 1-10 milliseconds). This is primarily useful for extraordinarily large data stores, accessed without human operators. Typical examples include tape libraries and optical jukeboxes. D. Off-line storage:
Off-line storage is computer data storage on a medium or a device that is not under the control of a processing unit. The medium is recorded, usually in a secondary or tertiary storage device, and then physically removed or disconnected. It must be inserted or connected by a human operator before a computer can access it again. Unlike tertiary storage, it cannot be accessed without human interaction. In modern personal computers, most secondary and tertiary storage media are also used for off-line storage. Optical discs and flash memory devices are most popular, and to much lesser extent removable hard disk drives.
In enterprise uses, magnetic tape is predominant. Older examples are floppy disks, Zip disks, or punched cards. IV. Characteristics of storage Storage technologies at all levels of the storage hierarchy can be differentiated by evaluating certain core characteristics as well as measuring characteristics specific to a particular implementation. These core characteristics are volatility, mutability, accessibility, and addressability. For any particular implementation of any storage technology, the characteristics worth measuring are capacity and performance. A. Volatility:
Non-volatile memory will retain the stored information even if it is not constantly supplied with electric power. It is suitable for long-term storage of information. Nowadays used for most of secondary, tertiary, and off-line storage. In 1950s and 1960s, it was also used for primary storage, in the form of magnetic core memory. Volatile memory requires constant power to maintain the stored information. The fastest memory technologies of today are volatile ones (not a universal rule). Since primary storage is required to be very fast, it predominantly uses volatile memory.
B. Mutability: Read/write storage or mutable storage allows information to be overwritten at any time. A computer without some amount of read/write storage for primary storage purposes would be useless for many tasks. Modern computers typically use read/write storage also for secondary storage. Read only storage retains the information stored at the time of manufacture, and write once storage (Write Once Read Many) allows the information to be written only once at some point after manufacture. These are called immutable storage.
Immutable storage is used for tertiary and off-line storage. Examples include CD-ROM and CD-R. C. Accessibility: Random access any location in storage can be accessed at any moment in approximately the same amount of time. Such characteristic is well suited for primary and secondary storage. Sequential access the accessing of pieces of information will be in a serial order, one after the other; therefore the time to access a particular piece of information depends upon which piece of information was last accessed. Such characteristic is typical of off-line storage. D. Addressability:
Location-addressable each individually accessible unit of information in storage is selected with its numerical memory address. In modern computers, location-addressable storage usually limits to primary storage, accessed internally by computer programs, since location-addressability is very efficient, but burdensome for humans. The underlying device is still location-addressable, but the operating system of a computer provides the file system abstraction to make the operation more understandable. In modern computers, secondary, tertiary and off-line storage use file systems. E.
Capacity: Raw capacity the total amount of stored information that a storage device or medium can hold. It is expressed as a quantity of bits or bytes (e. g. 10. 4 megabytes). Memory storage density the compactness of stored information. It is the storage capacity of a medium divided with a unit of length, area or volume (e. g. 1. 2 megabytes per square inch). F. Performance: Latency the time it takes to access a particular location in storage. The relevant unit of measurement is typically nanosecond for primary storage, millisecond for secondary storage, and second for tertiary storage.
It may make sense to separate read latency and write latency, and in case of sequential access storage, minimum, maximum and average latency. G. Energy use: Storage devices that reduce fan usage, automatically shut-down during inactivity, and low power hard drives can reduce energy consumption 90 percent. 2. 5 inch hard disk drives often consume less power than larger ones. Low capacity solid-state drives have no moving parts and consume less power than hard disks. Also, memory may use more power than hard disks. V. Fundamental storage technologies
As of 2008, the most commonly used data storage technologies are semiconductor, magnetic, and optical, while paper still sees some limited usage. Some other fundamental storage technologies have also been used in the past or are proposed for development. A. Semiconductor: Semiconductor memory uses semiconductor-based integrated circuits to store information. A semiconductor memory chip may contain millions of tiny transistors or capacitors. Volatile and non-volatile forms of semiconductor memory exist. In modern computers, primary storage almost exclusively consists of dynamic volatile semiconductor memory or dynamic random access memory.
Since the turn of the century, a type of non-volatile semiconductor memory known as flash memory has steadily gained share as off-line storage for home computers. Non-volatile semiconductor memory is also used for secondary storage in various advanced electronic devices and specialized computers. B. Magnetic: Magnetic storage uses different patterns of magnetization on a magnetically coated surface to store information. Magnetic storage is non-volatile. The information is accessed using one or more read/write heads which may contain one or more recording transducers.
A read/write head only covers a part of the surface so that the head or medium or both must be moved relative to another in order to access data. In modern computers, magnetic storage will take these forms: ¦ Magnetic disk ¦ Floppy disk, used for off-line storage ¦ Hard disk drive, used for secondary storage ¦ Magnetic tape data storage, used for tertiary and off-line storage In early computers, magnetic storage was also used for primary storage in a form of magnetic drum, or core memory, core rope memory, thin-film memory, twister memory or bubble memory.
Also unlike today, magnetic tape was often used for secondary storage. C. Optical: Optical storage, the typical optical disc, stores information in deformities on the surface of a circular disc and reads this information by illuminating the surface with a laser diode and observing the reflection. Optical disc storage is non-volatile. The deformities may be permanent (read only media), formed once (write once media) or reversible (recordable or read/write media). The following forms are currently in common use. CD, CD-ROM, DVD, BD-ROM: Read only storage, used for mass distribution of digital information (music, video, computer programs) ¦ CD-R, DVD-R, DVD+R, BD-R: Write once storage, used for tertiary and off-line storage ¦ CD-RW, DVD-RW, DVD+RW, DVD-RAM, BD-RE: Slow write, fast read storage, used for tertiary and off-line storage ¦ Ultra Density Optical or UDO is similar in capacity to BD-R or BD-RE and is slow write, fast read storage used for tertiary and off-line storage Magneto-optical disc storage is optical disc storage where the magnetic state on a ferromagnetic surface stores information.
The information is read optically and written by combining magnetic and optical methods. Magneto-optical disc storage is non-volatile, sequential access, slow write, fast read storage used for tertiary and off-line storage. D. Paper: Paper data storage, typically in the form of paper tape or punched cards, has long been used to store information for automatic processing, particularly before general-purpose computers existed. Information was recorded by punching holes into the paper or cardboard medium and was read mechanically (or later optically) to determine whether a particular location on the medium was solid or contained a hole.
A few technologies allow people to make marks on paper that are easily read by machine—these are widely used for tabulating votes and grading standardized tests. Barcodes made it possible for any object that was to be sold or transported to have some computer readable information securely attached to it. E. Uncommon: Vacuum tube memory, a William’s tube used a cathode ray tube, and a Selectron tube used a large vacuum tube to store information. These primary storage devices were short-lived in the market, since Williams tube was unreliable and Selectron tube was expensive.
Electro-acoustic memory also known as delay line memory used sound waves in a substance such as mercury to store information. Delay line memory was dynamic volatile, cycle sequential read/write storage, and was used for primary storage. Optical tape is a medium for optical storage generally consisting of a long and narrow strip of plastic onto which patterns can be written and from which the patterns can be read back. It shares some technologies with cinema film stock and optical discs, but is compatible with neither.
The motivation behind developing this technology was the possibility of far greater storage capacities than either magnetic tape or optical discs. Phase-change memory uses different mechanical phases of Phase Change Material to store information in an X-Y addressable matrix, and reads the information by observing the varying electrical resistance of the material. Phase-change memory would be non-volatile, random access read/write storage, and might be used for primary, secondary and off-line storage. Most rewritable and many write once optical disks already use phase change material to store information.
Holographic data storage stores information optically inside crystals or photopolymers. Holographic storage can utilize the whole volume of the storage medium, unlike optical disc storage which is limited to a small number of surface layers. Holographic storage would be non-volatile, sequential access, and either write once or read/write storage. It might be used for secondary and off-line storage. See Holographic Versatile Disc (HVD). Molecular memory stores information in polymer that can store electric charge. Molecular memory might be especially suited for primary storage.
The theoretical storage capacity of molecular memory is 10 terabits per square inch. Data storage tag (DST), also sometimes known as an archival tag is a data logger that uses sensors to record data at predetermined intervals. Data storage tags usually have a large memory size and a long lifetime. Most archival tags are supported by batteries that allow the tag to record positions for several years. Alternatively some tags are solar powered and allow the scientist to set their own interval; this then allows data to be recorded for significantly longer than battery-only powered tags.
Information repository is an easy way to deploy secondary tier of data storage that can comprise multiple, networked data storage technologies running on diverse operating systems, where data that no longer needs to be in primary storage is protected, classified according to captured metadata, processed, de-duplicated, and then purged, automatically, based on data service level objectives and requirements. In information repositories, data storage resources are virtualized as composite storage sets and operate as a federated environment.
Information repositories were developed to mitigate problems arising from data proliferation and eliminate the need for separately deployed data storage solutions because of the concurrent deployment of diverse storage technologies running diverse operating systems. They feature centralized management for all deployed data storage resources. They are self-contained, support heterogeneous storage resources, support resource management to add, maintain, recycle, and terminate media, track of off-line media, and operate autonomously. VI. Related technologies A. Network connectivity:
A secondary or tertiary storage may connect to a computer utilizing computer networks. This concept does not pertain to the primary storage, which is shared between multiple processors in a much lesser degree. Direct-attached storage (DAS) is a traditional mass storage that does not use any network. This is still a most popular approach. This term was coined lately, together with NAS and SAN. Network-attached storage (NAS) is mass storage attached to a computer which another computer can access at file level over a local area network, a private wide area network, or in the case of online file storage, over the Internet.
NAS is commonly associated with the NFS and CIFS/SMB protocols. Storage area network (SAN) is a specialized network that provides other computers with storage capacity. The crucial difference between NAS and SAN is the former presents and manages file systems to client computers, whilst the latter provides access at block-addressing (raw) level, leaving it to attaching systems to manage data or file systems within the provided capacity. SAN is commonly associated with Fiber Channel networks. B. Robotic storage:
Large quantities of individual magnetic tapes and optical or magneto-optical discs may be stored in robotic tertiary storage devices. In tape storage field they are known as tape libraries, and in optical storage field optical jukeboxes, or optical disk libraries per analogy. Smallest forms of either technology containing just one drive device are referred to as autoloaders or auto changers. Robotic-access storage devices may have a number of slots, each holding individual media, and usually one or more picking robots that traverse the slots and load media to built-in drives. The arrangement of the slots and picking devices affects erformance.
Important characteristics of such storage are possible expansion options: adding slots, modules, drives, robots. Tape libraries may have from 10 to more than 100,000 slots, and provide terabytes or petabytes of near-line information. Optical jukeboxes are somewhat smaller solutions, up to 1,000 slots. Robotic storage is used for backups, and for high-capacity archives in imaging, medical, and video industries. Hierarchical storage management is a most known archiving strategy of automatically migrating long-unused files from fast hard disk storage to libraries or jukeboxes. If the files are needed, they are retrieved back to disk.
References J. S. Vitter, Algorithms and Data Structures for External Memory. Series on Foundations and Trends in Theoretical Computer Science, now Publishers, Hanover, MA, 2008, ISBN 978-1-60198-106-6. National Communications System (1996). Federal Standard 1037C – Telecommunications: Glossary of Telecommunication Terms. Super Talent’s 2. 5″ IDE Flash hard drive – The Tech Report – Page 13. (http://techreport. com/articles. x/10334/13)
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