BIC 10503: COMPUTER ARCHITECTURE Chapter 3 Part 3 (External Memory)

Overview 3.3.1 Magnetic Disk 3.3.2 RAID 3.3.3 Solid State Drives 3.3.4 Optical Memory 3.3.5 Magnetic Tape

3.3.1 Magnetic Disk

Magnetic Disk Glass substrate are: Benefits of the glass substrate: The primary computer storage device. Bits are magnetically recorded and can be re-recorded over and over. A disk is a round platter. Constructed of nonmagnetic material, called the substrate, and coated with a thin layer (10-20nm) of a magnetic material Previously, the substrate was an aluminium or aluminium alloy material Recently glass substrates have been introduced Glass substrate are: More Smoother More Flatter More Thinner More Rigid Benefits of the glass substrate: Improved surface uniformity Increases reliability Lower flight heights (See later) Better stiffness Better shock/damage resistance

Inductive Write/Magnetoresistive Read Head

Magnetic Disk (cont) Bits of data (0’s and 1’s) are stored on magnetized regions on the platters. A disk rotates rapidly (& never stops). A disk head reads and writes bits of data as they pass under the head. Often, several platters are organized into a disk pack (or disk drive).

Magnetic Read and Write Mechanisms Recording & retrieval via conductive coil called a read/write 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

Magnetic Read and Write Mechanisms (cont) Separate read head, close to write head The read head consists of Magneto Resistive (MR) sensor The MR sensor detects Electrical Resistance depends on direction of magnetic field. Electrical resistance converted into voltage signals. The design of MR allows high frequency operation Results in higher storage density and speed

Data Organization Tracks Sectors InterSector Gap InterTrack Gap Data are organized on the platter in a concentric set of rings. Sectors Subdivision of a track. Can be fixed or variable length. InterSector Gap Spacing between sectors. For optimum precision. InterTrack Gap Spacing between tracks. To prevent/minimize head misalignment error. To avoid interference of magnetic fields.

Sectors Tracks Intersector Gap Intertrack Gap Disk Data Layout

Disk Layout Methods Diagram

Constant Angular Velocity (CAV) The number of sectors in each track is equal. The advantage: individual blocks of data can be directly addressed by track and sector. only takes a short movement of the head to a specific track short wait for the proper sector to spin under the head. The disadvantage: the amount of data that can be stored on the long outer tracks is the same as inner tracks.

Multiple Zoned Recording Placing more sectors in the outer tracks than in the inner tracks. Grouping the tracks into sets called zones. Tracks in the inner zones contain the fewest sectors, and tracks in the outer zones contain the most sectors. The advantage: Space on platter are better utilized. Allows for greater overall storage capacity The disadvantage: Require more complex circuitry.

Winchester Disk Developed by IBM Significant technique The read/write heads would rise or lift off from the disk while the disk speed increased Lowered the read/write heads when the disk decelerated. This enabled the heads to not have to move off the disk each time the speed was increased or decreased.

Physical Characteristics of Disk Systems

Characteristics Fixed-head disk Movable-head disk Double sided disk One read-write head per track Heads are mounted on a fixed ridged arm that extends across all tracks Removable disk Can be removed and replaced with another disk Advantages: Unlimited amounts of data are available with a limited number of disk systems A disk may be moved from one computer system to another Floppy disks and ZIP cartridge disks are examples of removable disks Movable-head disk One read-write head Head is mounted on an arm The arm can be extended or retracted Non-removable disk Permanently mounted in the disk drive The hard disk in a personal computer is a non-removable disk Double sided disk Magnetizable coating is applied to both sides of the platter

Multiple Platters Multiple platters stacked vertically a fraction of an inch apart. Multiple arms are provided. Movable head, with one read-write head per platter surface. All are mechanically fixed so that all are at the same distance from the center of the disk and move together. Thus, at any time, all of the heads are positioned over tracks that are of equal distance from the center of the disk.

Tracks Cylinders A cylinder is the set of tracks at a given radius of a disk pack. a cylinder is the set of tracks that can be accessed without moving the disk arm. All the information on a cylinder can be accessed without moving the read/write arm.

Estimating Capacities Track capacity = # of sectors per track * bytes per sector Cylinder capacity = # of tracks per cylinder * Track capacity Disk capacity = # of cylinders * cylinder capacity Number of cylinders = # of tracks in a surface

Estimating Capacities - Exercise Store a file of 20000 records on a disk with the following characteristics: # of bytes per sector = 512 # of sectors per track = 40 # of tracks per cylinder = 11 # of cylinders = 1331 Q1. How many cylinders does the file require if each data record requires 256 bytes? Q2. What is the total capacity of the disk?

Typical Hard Disk Parameters

Disk Performance Parameters To read/write the head must be positioned at the desired track and at the beginning of the desired sector on the track Read/Write Timing Seek time On a movable–head system, the time it takes to position the head at the track Rotational delay (rotational latency) The time it takes for the beginning of the sector to reach the head Access time The sum of the seek time and the rotational delay The time it takes to get into position to read or write Transfer time Once the head is in position, the read or write operation is then performed as the sector moves under the head This is the data transfer portion of the operation

3.3.2 RAID

Redundant Array of Independent Disks RAID Redundant Array of Independent Disks Consists of 7 levels (RAID 0 – 6) Three common characteristics: Set of physical disk drives viewed by the operating system as a single logical drive Data are distributed across the physical drives of an array in a scheme known as striping Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure

RAID 0 RAID 0 comprises striping (but no parity or mirroring). This level provides no data redundancy nor fault tolerance. But improves performance through parallelism of read and write operations across multiple drives. RAID 0 has no error detection mechanism, so the failure of one disk causes the loss of all data on the array

RAID 1 RAID 1 comprises mirroring (without parity or striping). Data are written identically to two (or more) drives, thereby producing a "mirrored set". The read request is serviced by any of the drives containing the requested data. This can improve read performance. Conversely, write performance can be degraded because all drives must be updated; thus the write performance is determined by the slowest drive.

RAID 2 RAID 2 comprises bit-level striping with dedicated Hamming-code parity. All disk rotation is synchronized and data is striped such that each sequential bit is on a different drive. Hamming-code parity is calculated across corresponding bits and stored on at least one parity drive.

RAID 3 RAID 3 is organized in a similar fashion to RAID 2. The difference is that RAID 3 requires only a single redundant disk, no matter how large the disk array.

RAID 4 In the case of RAID 4, the strips are relatively large. RAID 4 comprises block-level striping with dedicated parity. RAID 4 involves a write penalty when an I/O write request of small size is performed.

RAID 5 RAID 5 comprises block-level striping with distributed parity. Parity bit is distributed among the drives. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost. RAID 5 requires at least three disks.

RAID 6 RAID 6 comprises block-level striping with double distributed parity. Double parity provides fault tolerance up to two failed drives.

3.3.3 Solid State Drives

Flash Memory A type of semiconductor memory that are used in SSD. Used in many consumer electronic products including smart phones, GPS devices, MP3 players, digital cameras, and USB devices Cost and performance has evolved -> feasible to replace HDDs 2 types of flash memory: NOR NAND The basic unit of access is a bit Provides high-speed random access Used to store cell phone operating system code and on Windows computers for the BIOS program that runs The basic unit is 16 or 32 bits Reads and writes in small blocks Used in USB flash drives, memory cards, and in SSDs Does not provide a random-access external address bus so the data must be read on a block-wise basis

SSD Compared to HDD SSDs have the following advantages over HDDs: High-performance input/output operations per second (IOPS) Durability Longer lifespan Lower power consumption Quieter and cooler running capabilities Lower access times and latency rates

Practical Issues There are two practical issues peculiar to SSDs that are not faced by HDDs: SDD performance has a tendency to slow down as the device is used The entire block must be read from the flash memory and placed in a buffer Before the block can be written back to flash memory, the entire block of flash memory must be erased; it is not possible to erase just one page of the flash memory The entire block from the buffer is now written back to the flash memory Flash memory becomes unusable after a certain number of writes Typical limit is 100,000 writes Techniques for prolonging life: Embed the flash with a cache to reduce write operations Using wear-leveling algorithms that evenly distribute writes across block of cells Bad-block management techniques Most flash devices estimate their own remaining lifetimes so systems can anticipate failure and take preemptive action

3.3.4 Optical Memory Compact Disk CD-ROM CD-R CD-RW Digital Versatile Disk High-Definition Optical Disk

Compact Disk Read-Only Memory (CD-ROM) Data capacity for a CD-ROM is about 680MB Audio CD and the CD-ROM 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

CD Operation

CD-ROM Block Format • 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. Disc time (MIN,SEC). 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. • Data: User data. • Auxiliary: Additional user data in mode 2. In mode 1, this is a 288-byte error correcting code.

CD-ROM Appropriate for the distribution of large amounts of data to a large number of users The expense of the initial writing process it is not appropriate for individualized applications The CD-ROM advantages: 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 The 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

CD Recordable CD Rewritable (CD-R) (CD-RW) Can be repeatedly written and overwritten Disadvantage : the material eventually and permanently loses its desirable properties can cater between 500,000 and 1,000,000 erase cycles Advantage : it can be rewritten higher reliability and longer life Write-once read-many Accommodates applications in which only one or a small number of copies of a set of data is needed Provides a permanent record of large volumes of user data

Digital Versatile Disk (DVD) -Vast volumes of data can be crammed onto the disk, currently seven times as much as a CD-ROM -Replaced the videotape used in video cassette recorders (VCRs) -Replace the CD-ROM in personal computers and servers -Impressive picture quality -Can be randomly accessed like audio CDs

Digital Versatile Disk (DVD) Characteristics: 1. Bits are packed more closely on a DVD. 2. The DVD employs up to double layers of pits and lands. 3. The DVD-ROM can be two sided.

High-Definition Optical Disks Store high-definition content. Greater storage capacity compared to DVDs. Two competing technologies in market: HD DVD – can store 15GB on a single layer on a single side Blu-ray DVD - can store 25GB on a single layer - 3 versions are available [read only (BD-ROM), recordable once (BD-R) and rerecordable (BD-RE)]

3.3.5 Magnetic Tape

Magnetic Tape Slowest speed Lowest cost Sequential access device Same reading and recording techniques as disk systems Data on the tape are structured as a number of parallel tracks running lengthwise Serial recording  data laid out as a sequence of bits along each track Data read and written in contiguous blocks called physical records Typical recording technique used in serial tapes is known as serpentine recording

Magnetic Tape Features Serpentine recording : When data are being recorded, the first set of bits is recorded along the whole length of the tape. When the end of the tape is reached, the heads are repositioned to record a new track, and the tape is again recorded on its whole length, this time in opposite direction. The process continues back and forth, until the tape is full.

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