Chapter Goals

1. Chapter Goals • Describe the distinguishing characteristics of primary and secondary storage • Describe the devices used to implement primary storage • Describe memory allocation schemes • Compare and contrast secondary storage technology alternatives Systems Architecture, Fifth Edition

2. Chapter Goals (continued) • Describe factors that determine storage device performance • Choose appropriate secondary storage technologies and devices Systems Architecture, Fifth Edition

3. Systems Architecture, Fifth Edition

4. Systems Architecture, Fifth Edition

5. Characteristics of Storage Devices • Speed • Volatility • Access method • Portability • Cost and capacity • Let’s examine each of these... Systems Architecture, Fifth Edition

6. Speed • Primary storage speed • Typically faster than secondary storage speed by a factor of 105 or more • Expressed in nanoseconds (billionths of a second) • Secondary storage speed • Expressed in milliseconds (thousandths of a second) • Data transfer rate = 1 second/access time (in seconds) x unit of data transfer (in bytes) (simplified) Systems Architecture, Fifth Edition

7. Volatility • Primary storage devices are generally volatile • Cannot reliably hold data for long periods • Secondary storage devices are generally nonvolatile • Hold data without loss over long periods of time Systems Architecture, Fifth Edition

8. Access Method • Serial access (linear, such as a tape) • Random access (direct access, such as RAM) • Parallel access (simultaneous access, such as RAID) Systems Architecture, Fifth Edition

9. Portability • Removable storage media with standardized formats (e.g., compact disc and tape storage) • Typically results in slower access speeds Systems Architecture, Fifth Edition

10. Cost and Capacity • Cost increases: • With improved speed, volatility, or portability • As access method moves from serial to random to parallel access method • Primary storage - expensive (high speed and combination of parallel/random access methods) • Capacity of secondary storage devices is greater than primary storage devices Systems Architecture, Fifth Edition

11. Systems Architecture, Fifth Edition

12. Memory-Storage Hierarchy Systems Architecture, Fifth Edition

13. Primary Storage Devices • Critical performance characteristics • Access speed • Data transfer unit size • Must closely match CPU speed and word size to avoid wait states Systems Architecture, Fifth Edition

14. nonvolatile memory Storing Electrical Signals • Directly • By devices such as batteries and capacitors • Trade off between access speed and volatility • Indirectly • Uses energy to alter the state of a device; inverse process regenerates equivalent electrical signal • Modern computers use memory implemented with semiconductors (RAM and NVM) Systems Architecture, Fifth Edition

15. Random Access Memory • Characteristics • Microchip implementation using semiconductors • Ability to read and write with equal speed • Random access to stored bytes, words, or larger data units • Basic types • Static RAM (SRAM) – faster than DRAM; uses 6 transistors/bit; does not need refreshing; faster than DRAM • Dynamic RAM (DRAM) – uses 1 transistor and 1 capacitor / bit; cheaper than SRAM; needs refreshing; higher density; needs more power requirements Systems Architecture, Fifth Edition

16. Random Access Memory • To bridge performance gap between memory and microprocessors • Read-ahead memory access • Synchronous read operations • On-chip memory caches Systems Architecture, Fifth Edition

17. Nonvolatile Memory • Random access memory with long-term or permanent data retention • Usually relegated to specialized roles and secondary storage; slower write speeds and limited number of rewrites • Generations of devices (ROM, EPROM, and EEPROM) Systems Architecture, Fifth Edition

18. Nonvolatile Memory • Flash RAM (most common NVM) • Competitive with DRAM in capacity and read performance • Relatively slow write speed • Limited number of write cycles (more on this later) • NVM technologies under development • Ferroelectric RAM • Polymer memory Systems Architecture, Fifth Edition

19. Memory Packaging • Dual in-line packages (DIPs) • Early RAM and ROM circuits • Single in-line memory module (SIMM) • Standard RAM package in late 1980s • Double in-line memory module (DIMM) • Newer packaging standard • A SIMM with independent electrical contacts on both sides of the module Systems Architecture, Fifth Edition

20. Systems Architecture, Fifth Edition

21. CPU Memory Access • Critical design issues for primary storage devices and processors • Physical organization of memory • Organization of programs and data within memory • Method(s) of referencing specific memory locations Systems Architecture, Fifth Edition

22. Physical Memory Organization • Physical memory • Actual number of memory bytes that physically are installed in the machine • Most and least significant bytes • Big endian (stores most significant byte/bit at lowest memory address) and little endian • Addressable memory • Highest numbered storage byte that can be represented Systems Architecture, Fifth Edition

23. Systems Architecture, Fifth Edition

24. Systems Architecture, Fifth Edition

25. Memory Access Time • Memory rated at PCxxxx delivers peak bandwidth of xxxxMB/sec. • For example, PC3200 memory delivers 3.2GB/sec peak bandwidth. • Typically, there is a significant delay before you can get back the first byte (say 6 cycles) • Random accesses would deliver around 500KB/sec. Systems Architecture, Fifth Edition

26. Memory compared to CPU • How much data can a CPU pump around? • 2GHz = 2,000,000 cycles per second • Each cycle, the CPU can move around a word(4 bytes on a 32-bit machine) • so 8GB/sec Systems Architecture, Fifth Edition

27. Magnetic Storage • Exploits duality of magnetism and electricity • Converts electrical signals into magnetic charges • Captures magnetic charge on a storage medium • Later regenerates electrical current from stored magnetic charge • Polarity of magnetic charge represents bit values zero and one Systems Architecture, Fifth Edition

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29. Systems Architecture, Fifth Edition

30. Systems Architecture, Fifth Edition

31. Magnetic Tape • Ribbon of plastic with a coercible (usually metallic oxide) surface coating • Mounts in a tape drive for reading and writing • Relatively slow serial access • Compounds magnetic leakage; wraps upon itself • Susceptible to stretching, friction, temperature variations Systems Architecture, Fifth Edition

32. Systems Architecture, Fifth Edition

33. Magnetic Tape • Two approaches to recording data • Linear recording • Helical scanning • Several formats and standards (e.g., DDS [DAT], AIT, Mammoth, DLT (digital linear tape), LTO (linear tape-open)) • Highest capacity tapes hold about 1 TB (same as largest disk drives) • But much cheaper per byte Systems Architecture, Fifth Edition

34. Systems Architecture, Fifth Edition

35. Modern Tape Formats and Capacities (uncompressed) • DDS (DAT), from Sony and HP: 2-36 GB • AIT, from Sony: 35-400 GB • Mammoth, from Exabyte: 20-80 GB • DLT, from Quantum: 20-600 GB • LTO, from HP, IBM, Seagate: 100-800 GB Systems Architecture, Fifth Edition

36. Magnetic Disk • Flat, circular platter with metallic coating that is rotated beneath read/write heads • Random access device; read/write head can be moved to any location on the platter • Hard disks and floppy disks • Cost performance leader for general-purposeon-line secondary storage Systems Architecture, Fifth Edition

37. Systems Architecture, Fifth Edition

38. Systems Architecture, Fifth Edition

39. Magnetic Disk Access Time • Head-to-head switching time • Track-to-track seek time • Rotational delay • Most important performance numbers • Average access time • Sequential access time • Sustained data transfer rate Systems Architecture, Fifth Edition

40. Average Access Time • Head switching time (negligible) • Plus head seek time (given in ms) • Plus rotational delay (on average, ½ a turn)If disk spins at 6000RPM, what is the rotational delay? • One turn takes 1/6000 min or 1/100 sec = 10ms • ½ turn takes 5ms. Systems Architecture, Fifth Edition

41. Average Access Time • Plus read time (time to spin an entire sector) • If the drive spins at 6000RPM and the disk has 20 sectors per track, what is the read time? • Time for 1 full spin is • Time for 1/20 of a spin is Systems Architecture, Fifth Edition

42. Let’s Try Another Problem • Drive spins at 7200RPM and has average seek time of 8ms. The disk has 24 sectors per track. What is the average access time? • Head seek time = 0.008 sec (given) • Rotational delay (1/2 spin) = 7200 RPM (1/2) = 120 RPS(1/2) = 1/120 sec/rev(1/2) = 0.0042 sec • Read time (1 sector) = 0.0084 (full spin) / 24 sectors per track = 0.00035 sec • Total = 0.008 + 0.0042 + 0.00035 = 0.01255 sec or 12.55 ms • This is the average access time Systems Architecture, Fifth Edition

43. Sequential Access Time • The best possible read time is if the head is in exactly the right place (like when we read consecutive sectors) • Sequential access time is the amount of time for one sector to spin under the head • Drive spins at 7200RPM and has average seek time of 8ms. The disk has 24 sectors per track. What is the sequential access time? Systems Architecture, Fifth Edition

44. Sequential Access Time • Drive spins at 7200RPM and has average seek time of 8ms. The disk has 24 sectors per track. What is the sequential access time? Read time (1 sector) = 0.0084 (full spin) / 24 sectors per track = 0.00035 sec (no need to include head seek time and rotational delay) Systems Architecture, Fifth Edition

45. Data Transfer Rate • The data transfer rate is the number of access that can be made per second times the amount of data per transfer. • The size of a transfer is the size of a sector, typically 512 bytes. Systems Architecture, Fifth Edition

46. Maximum Data Transfer Rate • Maximum data transfer rate is computed using the best possible access time (sequential access time). • In the prior example, sequential access time was 0.35ms = 0.00035 seconds • Accesses per second is • Transfer rate is Systems Architecture, Fifth Edition

47. Sustained Data Transfer Rate • Sustained data transfer rate is computed using the average access time (sectors may not be contiguous). • In the prior example, average access time was 12.55ms = 0.01255 seconds • Accesses per second is • Transfer rate is Systems Architecture, Fifth Edition

48. Systems Architecture, Fifth Edition

49. To increase capacity per platter, disk manufacturers divide tracks into zones and vary the sectors per track in each zone. Systems Architecture, Fifth Edition

50. Optical Mass Storage Devices • Store bit values as variations in light reflection • Higher areal density and longer data life than magnetic storage • Standardized and relatively inexpensive • Uses: read-only storage with low performance requirements, applications with high capacity requirements, and where portability in a standardized format is needed Systems Architecture, Fifth Edition