Magnetic Disk

3. Magnetic Disk • Magnetic disks are the foundation of external memory on virtually all computer systems. • A disk is a circular platter constructed of nonmagnetic material, called the substrate, coated with a magnetizable material. • The substrate has been an aluminum material. • Glass substrates have been introduced.

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

5. Read and Write Mechanisms • 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

7. Data Organization and Formatting • Concentric set of rings, called tracks each track has the same width as the head. There are thousands of tracks per surface. - 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 (track)

9. Disk Velocity • Bit near centre of rotating disk passes fixed point slower than bit on outside of disk • Increase spacing between bits in different tracks • Rotate disk at constant angular velocity (CAV) - Gives pie shaped sectors and concentric tracks - Individual tracks and sectors addressable - Move head to given track and wait for given sector - Waste of space on outer tracks - Lower data density • Can use zones to increase capacity - Each zone has fixed bits per track -More complex circuitry

11. Finding Sectors • Must be able to identify start of track and sector • Format disk - Additional information not available to user - Marks tracks and sectors • An example of disk formatting is shown in Figure 6.4. In this case, each track contains 30 fixed-length sectors of 600 bytes each. • Each sector holds 512 bytes of data plus control information useful to the disk controller.

12. The ID field is a unique identifier or address used to locate a particular sector. The SYNCH byte is a special bit pattern that delimits the beginning of the field. The track number identifies a track on a surface. • The head number identifies a head, because this disk has multiple surfaces. • The ID and data fields each contain an error detecting code.

15. Physical Characteristics • Head Motion - Fixed head (one read write head per track and Heads mounted on fixed ridged arm) - Movable head(one per surface and mounted on a movable arm). • Disk Portability - Nonremovable disk (fixed ) - Removable disk (Can be removed from drive and replaced with another disk, provides unlimited storage capacity, and easy data transfer between systems)

16. Physical Characteristics • Sides - Single sided - double (usually) sided • Platters - Single platters - multiple platter (One head per side, heads are joined and aligned, aligned tracks on each platter form cylinders and data is striped by cylinder: 1. Reduces head movement 2. Increases speed (transfer rate)

19. Physical Characteristics • Head mechanism - Contact (Floppy disk) 8”, 5.25”, 3.5” Small capacity up to 1.44Mbyte (2.88M never popular) Slow, universal, cheap - Fixed gap - Flying (Winchester) Developed by IBM in Winchester (USA), Sealed unit One or more platters (disks), Very small head to disk gap universal, cheap, Fastest external storage Getting larger all the time. 250 Gigabyte now easily available

20. RAID • Redundant Array of Independent Disks (RAID • Redundant Array of Inexpensive Disks • 7 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

21. RAID These levels share three common characteristics: 1. RAID is a set of physical disk drives viewed by the operating system as a single logical drive. 2. Data are distributed across the physical drives of an array in a scheme known as striping, described subsequently. • 3. Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure. • The details of the second and third characteristics differ for the different RAID levels. RAID 0 and RAID 1 do not support the third characteristic.

22. 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

24. 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

26. RAID 2 • RAID levels 2 and 3 make use of a parallel access technique. • In a parallel access array, all member disks participate in the execution of every I/O request • the individual drives are synchronized so that each disk head is in the same position on each disk at any given time. • RAID 2 requires fewer disks than RAID 1

27. RAID 3 • RAID 3 requires only a single redundant disk. • Employs parallel access, with data distributed in small strips • Can achieve very high data transfer rates. • Only one I/O request can be executed at a time

28. 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

30. 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

31. 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, because each write affects two parity blocks.

36. Compact Disk CD • Both the audio CD and the CD-ROM (compact disk read-only memory) share a similar technology. The main difference is that CD-ROM players are more rugged and have error correction devices to ensure that data are properly transferred from disk to computer. • Information is retrieved from a CD or CD-ROM by a low-powered laser housed in an optical-disk player, or drive unit.

37. Optical Storage CD-ROM • Originally for audio • 650Mbytes giving over 70 minutes audio • Polycarbonate coated with highly reflective coat, usually aluminum • Data stored as pits • Read by reflecting laser • Constant packing density The areas between pits are called lands. A land is a smooth surface, which reflects back at higher intensity • Constant linear velocity ( CLV )

38. CD Operation • Data on the CD-ROM are organized as a sequence of blocks. A typical block format is shown in Figure 6.11. It consists of the following fields: • Sync : The sync field identifies the beginning of a block. It consists of a byte of all 0s, 10 bytes of all 1s, and a byte of all 0s. • Header: The header contains the block address and the mode byte. - Mode 0 specifies a blank data field; - mode 1 specifies the use of an error-correcting code and 2048 bytes of data; - mode 2 specifies 2336 bytes of user data with no error-correcting code.

39. Data: User data. • Auxiliary: Additional user data in mode 2. In mode 1, this is a 288-byte error correcting code.

41. CD-ROM advantage • The optical disk together with the information stored on it can be mass replicated inexpensively • The optical disk is removable, allowing the disk itself to be used for archival storage • CD-ROM disadvantages: • It is read-only and cannot be updated. • It has an access time much longer than that of a magnetic disk drive, as much as half a second

42. CD-ROM Drive Speeds • Audio is single speed - Constant linear 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)

44. Other Optical Storage • CD-Recordable (CD-R) - write-once read-many CD - Is attractive for archival storage of documents and files. - It provides a permanent record of large volumes of user data. • CD-RW - It can be repeatedly written and overwritten - Erasable - Getting cheaper - Phase change disk uses material has two different reflectivities in two different phase states

45. Digital Versatile Disk - Used to indicate a computer drive - Will read computer disks and play video disks • DVD – technology - Multi-layer - Very high capacity (4.7G per layer) - Full length movie on single disk - Using MPEG compression - Finally standardized (honest!) - Movies carry regional coding - Players only play correct region films - Can be “fixed

46. The DVD’s greater capacity is due to three differences from CDs (Figure 6.12): • Bits are packed more closely on a DVD. • The DVD employs a second layer of pits and lands on top of the first layer. • The DVD-ROM can be two sided, whereas data are recorded on only one side of a CD. This brings total capacity up to 17 GB.

48. 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

49. 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)

50. 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