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Chapter 5 Data Storage Technology
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Chapter 5 Data Storage Technology

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  1. Chapter 5Data Storage Technology

  2. Chapter Goals • Characteristics of primary and secondary storage • Devices used to implement primary storage • Memory allocation schemes • Compare and contrast secondary storage technology alternatives • Factors that determine storage device performance • Choose appropriate secondary storage technologies and devices INFO 225: Chapter 5

  3. Storage Device Characteristics INFO 225: Chapter 5

  4. Storage Device Characteristics • Speed • Volatility • Access method • Portability • Cost and capacity INFO 225: Chapter 5

  5. Storage Device Characteristics • Speed • The delay between a user request for program execution and the first prompt for the user input depends on the speed of primary and secondary storage devices. • Primary storage speed is typically greater than secondary storage speed by a factor of 105 or more. • RAM cannot be read/written in one CPU cycle (i.e. at the same time). • Wait state is a CPU cycle spent waiting for a storage access or I/O operation to end. INFO 225: Chapter 5

  6. Storage Device Characteristics 1. Speed • Storage device speed is called access time. • Access time is the time required for one complete read or write operation. • The access time for reading and writing is assumed to be the same unless otherwise stated. • Access time for all storage location of RAM is the same. • Access time for different storage locations of secondary storage devices are different, therefore an average access time is expressed. INFO 225: Chapter 5

  7. Storage Device Characteristics 1. Speed • A complete measure of data access speed consists of access time and the unit of data transfer: • Unit of data transfer for RAM is WORD which depends on CPU (2,4, 8, etc.) • Unit of data transfer for secondary storage is Block (sector in hard drives; 512 Bytes by default) • Example: • What is data transfer rate in (mega byte per seconds) of a RAM if its Access time is 20 ns & Word size is 64 bits? • Answer: (1sec/20ns)x(64bits/8)=400 MBps INFO 225: Chapter 5

  8. Storage Device Characteristics 1. Speed • Zero wait state: is when Data is readily available for CPU and no CPU cycle is wasted. • Example: What is memory access time for a 3.0 GHz CPU with zero wait state: • Answer: Cycle time = (1/3.0 GHz) = 0.33 ns • Currently DRAM has Access time about ~ 17 ns • The fastest DRAM is at least 50 times slower than CPU INFO 225: Chapter 5

  9. Storage Device Characteristics 2. Volatility • A storage device or medium is non-volatile if it holds data without loss over long periods of time. • A storage device or medium is volatile if it cannot reliably hold data for long periods of time. INFO 225: Chapter 5

  10. Storage Device Characteristics 2. Volatility • Primary storage devices are generally volatile. • Secondary storage devices are generally non-volatile. INFO 225: Chapter 5

  11. Storage Devices Characteristics 3. Access Methods: • Serial Access • stores and retrieve data items in a linear, or sequential order. (Magnetic tape) • Random Access(Direct Access) • access device is not restricted to any specific order when accessing data. (Hard Disk, RAM) • Parallel Access • a device that is capable of simultaneously accessing multiple storage locations. (Random Access Memory) INFO 225: Chapter 5

  12. Disk Pack Several platters Airtight, sealed module Mount disk pack on disk drive INFO 225: Chapter 5

  13. RAID Redundant Array of Independent Disks INFO 225: Chapter 5

  14. RAID0 INFO 225: Chapter 5

  15. RAID Characteristics/AdvantagesRAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive I/O performance is greatly improved by spreading the I/O load across many channels and drives Best performance is achieved when data is striped across multiple controllers with only one drive per controller No parity calculation overhead is involved Very simple design Easy to implement DisadvantagesNot a "True" RAID because it is NOT fault-tolerant The failure of just one drive will result in all data in an array being lost Should never be used in mission critical environments Recommended ApplicationsVideo Production and Editing Image Editing Pre-Press Applications Any application requiring high bandwidth INFO 225: Chapter 5

  16. Storage Device Characteristics 4. Portability • Data can be made portable by storing it on a removable storage medium or device. • Portable devices typically have slower access speed than permanently installed devices and those with non-removable media. INFO 225: Chapter 5

  17. Data DestroyedHead Crash INFO 225: Chapter 5

  18. Storage Device Characteristics 5. Cost and Capability • An increase in speed, permanence or portability generally comes at increased cost if all other factors are held constant. INFO 225: Chapter 5

  19. Storage Device Characteristics INFO 225: Chapter 5

  20. Storage Device Characteristics Memory-Storage Hierarchy • Cost and access speed generally decrease as one moves down the hierarchy. • Due to lower cost, capacity tends to increase as one moves down the hierarchy. INFO 225: Chapter 5

  21. Storage Device Characteristics INFO 225: Chapter 5

  22. Primary Storage Devices • Storing Electrical Signals • Random Access Memory • Read-Only Memory • Memory Packaging INFO 225: Chapter 5

  23. Primary Storage Devices Storing Electrical Signals • Data is represented as electrical signals. • Digital signals are used to transmit data to and from devices attached to the system bus. • Storage devices must accept electrical signals as input and output. INFO 225: Chapter 5

  24. Primary Storage Devices Random Access Memory (RAM) refers to the primary storage devices and have the following characteristics: • Microchip implementation using semiconductors • Ability to read and write with equal speed • Random access to stored bytes, words, or larger data units RAM fabrication is similar to the CPU but has a lower Access Time (clock rate) because: • Reading/Writing bits in parallel requires additional circuitry • RAM & CPU are on separate chips, hence delays in moving data between the two chips INFO 225: Chapter 5

  25. Primary Storage Devices Random Access Memory Two basic types of memory and several variation on each: • Static RAM (SRAM) • Dynamic RAM (DRAM) • Synchronous DRAM (SDRAM) • Ferroelectric RAM INFO 225: Chapter 5

  26. Primary Storage Devices Static RAM • Implemented with transistors. • Basic unit of storage is a flip-flop circuit. • A flip-flop is an electrical circuit that remembers its last position. • One position represents 1, the other position represents 0. INFO 225: Chapter 5

  27. Primary Storage Devices INFO 225: Chapter 5

  28. Primary Storage Devices Dynamic RAM • Uses transistors and capacitors. • Lose their charge quickly. • Require a fresh infusion of power thousands of times per second. • Each refresh operation is called a refresh cycle. • DRAM cannot perform a read/write operation at the same time during refresh cycle. • DRAM is 10 times chipper than SRAM. • DRAM has less complex circuit, more memory cells can be packed into each chip (i.e. has higher density) • DRAM is 5 times slower than SRAM, typical access time is 50 ns for DRAM & 10ns for SRAM. INFO 225: Chapter 5

  29. Primary Storage Devices • SRAM or DRAM does not match current CPU clock rates: • e.g. for a 3 GHz CPU: • 1/3 GHz = 0.33 ns • For a 4 GHz CPU: • 1/4 GHz = 0.25 ns • SRAM ( 5 to 10 times) or DRAM (50 times) are slower than this clock rate. INFO 225: Chapter 5

  30. Primary Storage Devices • A number of technologies are used to bridge the performance gap between memory and microprocessors: • Read-ahead memory access • Synchronous read operations • On-chip memory caches INFO 225: Chapter 5

  31. Primary Storage Operations Read-ahead memory access • Programs usually access instructions and data items sequentially. • Read-ahead activates read/write circuitry for location n+1 during or after access to memory location n. • If CPU issues access command for location n+1 part of the work needed is already complete. INFO 225: Chapter 5

  32. Primary Storage Operations Synchronous read operations Also referred to as Synchronous DRAM (SDRAM) • Read-ahead RAM that uses the same clock pulse as the system bus. • Read and write operations are broken into a series of simple steps and each step can be completed in one bus clock cycle. • Current SDRAM match bus clock rates of 500MHz • New SDRAM technology Double data rate (DDR) or (DDR2) perform two read or write per bus cycle. INFO 225: Chapter 5

  33. Primary Storage Operations On-chip memory caches Also referred to as: Cashed DRAM or Enhanced DRAM(EDRAM) • A small amount of SRAM is placed in each DRAM device. • When a read request for a specific word is received, that word and several extra words are stored in the SRAM cache in anticipation of future sequential read requests. • If those requests are made, the data can be more quickly accessed from the SRAM cache. INFO 225: Chapter 5

  34. Primary Storage Devices Nonvolatile Memory Read-Only Memory – a random access memory device that can store data permanently or semipermanently. Instructions that reside in ROM are called firmware (e.g. System BIOS). INFO 225: Chapter 5

  35. Primary Storage Devices Nonvolatile Memory Read-Only Memory • Electronically Erasable Programmable Read-Only Memory (EEPROM) • Flash Memory INFO 225: Chapter 5

  36. Primary Storage Devices Nonvolatile Memory Electronically Erasable Programmable Read-Only Memory – can be programmed, erased, and reprogrammed by signals sent from and external control source, such as a CPU. Flash Memory – can be erased and rewritten more quickly. Disadvantages: Mildly destructive (100,000+ write), slow. INFO 225: Chapter 5

  37. Primary Storage Devices Nonvolatile Memory Two promising technology to overcome shortcomings of flash memory: Ferroelectric RAM Embeds iron or iron compounds within a microchip to store bits in much the same manner as old-fashioned core memory. Polymer Memory Uses a special plastic with electrical resistance that can be increased or decreased by an electrical field. Its advantages include low-cost materials, nondestructive read access, and individual memory cells with not transistors. Current research on polymer memory is focused on efficient methods for accessing individual memory cells and low-cost fabrications methods. INFO 225: Chapter 5

  38. Primary Storage Devices Memory Packaging • Dual In-line Packages (DIPs) • Single In-line Memory Module (SIMM) • Double In-line Memory Module (DIMM) INFO 225: Chapter 5

  39. Primary Storage Devices Memory Packaging: Dual in-line packages (DIP) Single in-line memory module (SIMM) Double in-line memory module (DIMM) Memory circuits are embedded within microchips and groups of chips are packed on a small circuit board that can be installed or removed easily. INFO 225: Chapter 5

  40. CPU Memory Access • Physical Memory Organization • Main memory can be regarded as a sequence of contiguous, or adjacent memory cells. Physical Storage Devices: Low Memory Address High Memory Address 02468ACE Big endian: Describes architectures that store the most significant byte at the lowest memory address. (02468ACE) Little endian: describes architecture that store the least significant byte at the lowest memory address. (CE8A4602) INFO 225: Chapter 5

  41. CPU Memory Access • Addressable memory of a CPU: • Is the highest numbered storage byte that can be represented. • Addressable memory is determined by the number of bits used to represent an address. • Example: if 32 bits is used to represent an address, then 232= 4,294,967,296B = 4GB • Physical memory: the actual number of memory bytes that physically are installed in the machine • Physical memory is smaller than addressable memory. INFO 225: Chapter 5

  42. CPU Memory Access • Memory Allocation and Addressing • describes the assignment of specific memory addresses to system software, application programs and data. A Simple Memory Allocation Scheme: INFO 225: Chapter 5

  43. Memory Allocation and Addressing Memory Addressing: • Absolute Addressing – describes memory address operands that refer to actual physical memory locations. • Relative Addressing (Indirect Addressing) – The CPU automatically computes physical memory addresses, by adding the program offset to all memory address operands before accessing memory.. INFO 225: Chapter 5

  44. Memory Allocation and Addressing INFO 225: Chapter 5

  45. Memory Allocation and Addressing INFO 225: Chapter 5

  46. Magnetic Storage Magnetic Storage • Exploit the duality of magnetism and electricity. • Electric current is used to generate a magnetic field. • A magnetic field can be used to generate electricity. INFO 225: Chapter 5

  47. Magnetic Storage INFO 225: Chapter 5

  48. Magnetic Storage Undesirable Characteristics of Magnetism Storage Media: • Magnetic decay • Magnetic leakage • Minimum threshold current for read operations • Storage medium coercivity • Long-term storage medium integrity INFO 225: Chapter 5

  49. Magnetic Storage: Table 5-2: Factors and Measures that Protect Data From Against Loss INFO 225: Chapter 5

  50. Magnetic Storage Magnetic Decay and Leakage Magnetic Decay – the tendency of magnetically charges particles to lose their charge over time. Magnetic Leakage – a decrease in the strength of individual bit charges. INFO 225: Chapter 5