Unit-III_External Memory.ppt
- 1. UNIT-III Memory Management- External Memory
- 2. 2 Course Content: • Internal Memory – Semiconductor Main memory – Error correction – Advanced DRAM organization • External Memory – Magnetic Disk, RAID, SSD, Optical memory, Magnetic tapes
- 3. 3 Magnetic Disk • Disk substrate coated with magnetizable material (iron oxide…rust) • Substrate used to be aluminium • Now glass —Improved surface uniformity – Increases reliability —Reduction in surface defects – Reduced read/write errors —Better stiffness —Better shock/damage resistance
- 4. 4 Read and Write Mechanisms • Recording & retrieval via conductive coil called a head • May be single read/write head or separate ones • During read/write, head is stationary, platter rotates • Write — Current through coil produces magnetic field — Pulses sent to head — Magnetic pattern recorded on surface below • Read (traditional) — Magnetic field moving relative to coil produces current — Coil is the same for read and write • Read (contemporary) — Separate read head, close to write head — Partially shielded magneto resistive (MR) sensor — Electrical resistance depends on direction of magnetic field — High frequency operation – Higher storage density and speed
- 6. 6 Data Organization and Formatting • Concentric rings or tracks —Gaps between tracks —Reduce gap to increase capacity —Same number of bits per track (variable packing density) —Constant angular velocity • Tracks divided into sectors • Minimum block size is one sector • May have more than one sector per block
- 7. 7 Characteristics • Fixed (rare) or movable head • Removable or fixed • Single or double (usually) sided • Single or multiple platter • Head mechanism —Contact (Floppy) —Fixed gap —Flying (Winchester)
- 8. 8 Fixed/Movable Head Disk • Fixed head —One read write head per track —Heads mounted on fixed ridged arm • Movable head —One read write head per side —Mounted on a movable arm
- 9. 9 Removable or Not • Removable disk —Can be removed from drive and replaced with another disk —Provides unlimited storage capacity —Easy data transfer between systems • Nonremovable disk —Permanently mounted in the drive
- 10. 10 Multiple Platter • One head per side • Heads are joined and aligned • Aligned tracks on each platter form cylinders • Data is striped by cylinder —reduces head movement —Increases speed (transfer rate)
- 13. 13 Speed • Seek time —Moving head to correct track • (Rotational) latency —Waiting for data to rotate under head • Access time = Seek + Latency • Transfer rate
- 14. 14 Timing of Disk I/O Transfer
- 15. 15 RAID • Redundant Array of Independent Disks • Redundant Array of Inexpensive Disks • 6 levels in common use • Not a hierarchy • Set of physical disks viewed as single logical drive by O/S • Data distributed across physical drives • Can use redundant capacity to store parity information
- 16. 16 RAID 0 • No redundancy • Data striped across all disks • Round Robin striping • Increase speed —Multiple data requests probably not on same disk —Disks seek in parallel —A set of data is likely to be striped across multiple disks
- 17. 17 RAID 1 • Mirrored Disks • Data is striped across disks • 2 copies of each stripe on separate disks • Read from either • Write to both • Recovery is simple —Swap faulty disk & re-mirror —No down time • Expensive
- 18. 18 RAID 2 • Disks are synchronized • Very small stripes —Often single byte/word • Error correction calculated across corresponding bits on disks • Multiple parity disks store Hamming code error correction in corresponding positions • Lots of redundancy —Expensive —Not used
- 19. 19 RAID 3 • Similar to RAID 2 • Only one redundant disk, no matter how large the array • Simple parity bit for each set of corresponding bits • Data on failed drive can be reconstructed from surviving data and parity info • Very high transfer rates
- 20. 20 RAID 4 • Each disk operates independently • Good for high I/O request rate • Large stripes • Bit by bit parity calculated across stripes on each disk • Parity stored on parity disk
- 21. 21 RAID 5 • Like RAID 4 • Parity striped across all disks • Round robin allocation for parity stripe • Avoids RAID 4 bottleneck at parity disk • Commonly used in network servers
- 22. 22 RAID 6 • Two parity calculations • Stored in separate blocks on different disks • User requirement of N disks needs N+2 • High data availability —Three disks need to fail for data loss —Significant write penalty
- 23. 23 RAID 0, 1, 2
- 24. 24 RAID 3 & 4
- 25. 25 RAID 5 & 6
- 26. 26 Data Mapping For RAID 0
- 27. 27 Optical Storage CD-ROM • Originally for audio • 650Mbytes giving over 70 minutes audio • Polycarbonate coated with highly reflective coat, usually aluminium • Data stored as pits • Read by reflecting laser • Constant packing density • Constant linear velocity
- 28. 28 CD Operation
- 29. 29 CD-ROM Drive Speeds • Audio is single speed —Constant linier velocity —1.2 ms-1 —Track (spiral) is 5.27km long —Gives 4391 seconds = 73.2 minutes • Other speeds are quoted as multiples • e.g. 24x • Quoted figure is maximum drive can achieve
- 30. 30 CD-ROM Format • Mode 0=blank data field • Mode 1=2048 byte data+error correction • Mode 2=2336 byte data
- 31. 31 Random Access on CD-ROM • Difficult • Move head to rough position • Set correct speed • Read address • Adjust to required location
- 32. 32 CD-ROM for & against • Large capacity (?) • Easy to mass produce • Removable • Robust • Expensive for small runs • Slow • Read only
- 33. 33 Other Optical Storage • CD-Recordable (CD-R) —WORM —Now affordable —Compatible with CD-ROM drives • CD-RW —Erasable —Getting cheaper —Mostly CD-ROM drive compatible —Phase change – Material has two different reflectivities in different phase states
- 34. 34 CD and DVD
- 35. 35 High Definition Optical Disks • Designed for high definition videos • Much higher capacity than DVD — Shorter wavelength laser – Blue-violet range — Smaller pits • HD-DVD — 15GB single side single layer • Blue-ray — Data layer closer to laser – Tighter focus, less distortion, smaller pits — 25GB on single layer — Available read only (BD-ROM), Recordable once (BR-R) and re- recordable (BR-RE)
- 37. 37 Magnetic Tape • Serial access • Slow • Very cheap • Backup and archive • Linear Tape-Open (LTO) Tape Drives —Developed late 1990s —Open source alternative to proprietary tape systems
- 38. 38