Computer data storage
Even the first computer designs, Charles Babbage's Analytical Engine and Percy Ludgate's Analytical Machine, clearly distinguished between processing and memory (Babbage stored numbers as rotations of gears, while Ludgate stored numbers as displacements of rods in shuttles).
Without a significant amount of memory, a computer would merely be able to perform fixed operations and immediately output the result.
[1]: 20 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.
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 0 or 1.
For example, the complete works of Shakespeare, about 1250 pages in print, can be stored in about five megabytes (40 million bits) with one byte per character.
Data are encoded by assigning a bit pattern to each character, digit, or multimedia object.
By adding bits to each encoded unit, redundancy allows the computer to detect errors in coded data and correct them based on mathematical algorithms.
The cyclic redundancy check (CRC) method is typically used in communications and storage for error detection.
This utilizes substantially less storage (tens of percent) for many types of data at the cost of more computation (compress and decompress when needed).
Generally, the lower a storage is in the hierarchy, the lesser its bandwidth and the greater its access latency is from the CPU.
Historically, early computers used delay lines, Williams tubes, or rotating magnetic drums as primary storage.
Operating systems borrow RAM capacity for caching so long as it's not needed by running software.
[3] Spare memory can be utilized as RAM drive for temporary high-speed data storage.
Many types of "ROM" are not literally read only, as updates to them are possible; however it is slow and memory must be erased in large portions before it can be re-written.
Standard computers do not store non-rudimentary programs in ROM, and rather, use large capacities of secondary storage, which is non-volatile as well, and not as costly.
Rotating optical storage devices, such as CD and DVD drives, have even longer access times.
To reduce the seek time and rotational latency, data are transferred to and from disks in large contiguous blocks.
Another way to reduce the I/O bottleneck is to use multiple disks in parallel to increase the bandwidth between primary and secondary memory.
It is primarily used for archiving rarely accessed information since it is much slower than secondary storage (e.g. 5–60 seconds vs. 1–10 milliseconds).
When the computer has finished reading the information, the robotic arm will return the medium to its place in the library.
Off-line storage increases general information security since it is physically inaccessible from a computer, and data confidentiality or integrity cannot be affected by computer-based attack techniques.
Some systems, for example EMC Symmetrix, have integrated batteries that maintain volatile storage for several minutes.
Vulnerabilities that can instantly lead to total loss are head crashing on mechanical hard drives and failure of electronic components on flash storage.
[22] As of 2011[update], the most commonly used data storage media are semiconductor, magnetic, and optical, while paper still sees some limited usage.
Non-volatile semiconductor memory is also used for secondary storage in various advanced electronic devices and specialized computers that are designed for them.
As early as 2006, notebook and desktop computer manufacturers started using flash-based solid-state drives (SSDs) as default configuration options for the secondary storage either in addition to or instead of the more traditional HDD.
Barcodes make it possible for objects that are sold or transported to have some computer-readable information securely attached.
If a database cannot tolerate even such a smaller probability of data loss, then the RAID group itself is replicated (mirrored).
Large quantities of individual magnetic tapes, and optical or magneto-optical discs may be stored in robotic tertiary storage devices.
Important characteristics of such storage are possible expansion options: adding slots, modules, drives, robots.
