Chapter 5 External Memory

Contents • Magnetic Disk • RAID • Optical Memory • Magnetic Type 22

Magnetic Disk Data Organization and Formatting Physical Characteristics Disk Performance Parameters 22

Magnetic Disk Magnetic disk Disk is a circular platter constructed of metal or plastic coated with a magnetizable material Data are recorded on and later retrieved from the disk via a conducting coil named head 22

Data Organization & Formatting Magnetic disk Tracks Organization of data on the platter in a concentric set of ring Adjacent tracks are separated by gaps Density in the bits per linear inch, increases in moving from the outermost track to the innermost track 26

Data Organization & Formatting Magnetic disk Data are transferred to and from the disk in blocks Data are stored in block-size regions known as sectors There are typically between 10 and 100 sectors per track, and these may be of either fixed or variable length 26

Disk Data Layout Magnetic disk 31

Winchester Disk Format Magnetic disk Gap1 Gap1 Id Gap2 Data Gap3 Id Gap2 Data Gap3 Sync Byte Track Sync Byte Head Sector CRC Data CRC

Physical Characteristics Magnetic disk Head Motion Fixed head (one per track) Disk Portablilty Nonremovable disk Sides single sided double sided Platters Single platter Multiple platter Head Mechanism Contact (floppy) Aerodynamic gap(Winchester)

Physical Characteristics Magnetic disk Fixed or Movable head 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

Physical Characteristics Magnetic disk 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

Fixed-and Movable-Head Disks Magnetic disk

Multiple Platter One head per side Heads are joined and aligned Magnetic disk 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)

Multiple-Platter Disk Magnetic disk

Floppy Disk 8”, 5.25”, 3.5” Small capacity Slow, flexible platter Magnetic disk 8”, 5.25”, 3.5” Small capacity Up to 1.44Mbyte (2.88M never popular) Slow, flexible platter Universal Cheap

Winchester Hard Disk Developed by IBM in Winchester (USA) Sealed unit Magnetic disk Developed by IBM in Winchester (USA) Sealed unit One or more platters (disks) Heads fly on boundary layer of air as disk spins Very small head to disk gap Getting more robust Universal Cheap Fastest external storage Getting larger all the time Multiple Gigabyte now usual

Disk Performance Parameters Magnetic disk When the disk drive is operating, the disk is rotating at constant speed To read or write, the head must be positioned at the desired track and at the beginning of the desired sector on that track Track selection involves moving the head in a movable-head system or electronically selecting one head on a fixed-head system Seek time time it takes to position the head at the track is known as seek time

Disk Performance Parameters Magnetic disk Rotational delay The time it takes for the beginning of the sector to reach the head Access time Sum of the seek time, if any, and the rotational delay A technique known as rotational positional sensing(RPS) is used When the seek command has been issued, the channel is released to handle other I/O operations the seek is completed, the device determines when the data will rotate under the head

Timing of a Disk I/O Transfer Magnetic disk

Disk Performance Parameters Magnetic disk Seek Time Time it takes to position the head at the track Moving head to correct track Approximate seek time formula Ts = m x n + s Ts = estimated seek time n = number of track traversed m = constant that depends on the disk drive s = startup time 33

Disk Performance Parameters Magnetic disk Rotational Delay Time it takes for the beginning of the sector to reach the head Waiting for data to rotate under head Transfer Time T = b/(rN) T = transfer time B = number of bytes to be transferred N = number of bytes on a track R = rotation speed, in revolutions per second

Disk Performance Parameters Magnetic disk Access time = Seek + Rotational delay Time it takes to get into position to read or write Total average access time Ta = Ts + 1/2r + b/rN (Ts : average seek time)

RAID Rate in performance improvement Magnetic disk Rate in performance improvement Secondary storage << processor, main memory Disk storage system is the main focus in improving overall computer system performance Redundant Array of Independent Disks (RAID) 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 Redundant Array of Inexpensive Disks 6 levels in common use Not a hierarchy

RAID Levels Magnetic disk

RAID Level 0 It does not include redundancy to improve performance Magnetic disk It does not include redundancy to improve performance Data are striped across the available disks A set of logically consecutive strips that maps exactly one strip to each array member is referred to as stripe If a single I/O request consists of multiple logically contiguous strips then up to n strips for that request can be handled in parallel, greatly reducing the I/O transfer time

RAID 0(Nonredundant) Magnetic disk

Data Mapping for RAID Level 0 Array Magnetic disk

RAID Level 1 Mirrored Disks Data is striped across disks Magnetic disk 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

RAID 1(Mirrored) Magnetic disk

RAID Level 2 Disks are synchronized Very small stripes Magnetic disk 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

RAID 2 Magnetic disk

RAID Level 3 Similar to RAID 2 Magnetic disk 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

RAID 3 Magnetic disk

RAID Level 4 Each disk operates independently Magnetic disk 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

RAID 4(Block-Level Parity) Magnetic disk

RAID Level 5 Like RAID 4 Parity striped across all disks Magnetic disk 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 N.B. DOES NOT MEAN 5 DISKS!!!!!

RAID 5 Magnetic disk

RAID Level 6 Magnetic disk Two different parity calculations carried out and stored in separate blocks on different disks User data required N disks consists of N_2 disks. P, Q : two different data check algorithms Regenerate data even if two disks containing user data fail. Advantage if RAID 6 High data availability

RAID 6(Dual Redundancy) Magnetic disk

Optical Memory CD-ROM WORM Erasable Optical Disk Digital Video Disk (DVD) Magneto-Optical Disks

CD-ROM Originally for audio 650Mbytes giving over 70 minutes audio Optical memory 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 Constant linear velocity

CD-ROM Audio is single speed Other speeds are quoted as multiples Optical memory 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 The quoted figure is the maximum the drive can achieve

Optical Disk Products Optical memory

Disk Layout Method Optical memory (a) Constant angular velocity

Disk Layout Method Optical memory (b) Constant linear velocity

CD-ROM Format Sync : beginning of a block Optical memory Sync : beginning of a block Header : block address and mode of byte Mode 0=blank data field Mode 1=2048 byte data+error correction Mode 2=2336 byte data Data : user data Auxiliary : Additional user data in mode 2. 00 FF x 10 Min Sec Sector Mode Data Layered ECC 12 byte Sync 4 byte Id 2048 byte 288 byte 2352 byte

CD ROM Advantage & Disadvantage Optical memory Three major advantages of CD-ROM : Storage capacity is much greater on the optical disk Optical disk can be mass replicated inexpensively Optical disk is removable, allowing the disk itself to be used for archival storage Disadvantages of CD-ROM : Read only and cannot be updated Access time much longer than that of a magnetic disk drive, as much as half a second

WORM Write-once read-many CD Optical memory Write-once read-many CD More expensive disk controller than CD-ROM CD-writable

Erasable Optical Disk Can be repeatedly written and overwritten. Optical memory Can be repeatedly written and overwritten. Can be used secondary storage CD-RW Advantages High capacity Portability Reliability

Digital Video Disk Used to indicate a player for movies Multi-layer Optical memory Used to indicate a player for movies Only plays video disks 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”

Magnetic Tape Serial access Slow Very cheap Backup and archive

Nine-Track Magnetic Disk Format Magnetic tape