Computer Architecture Chapter (6): External Memory Eng. Nader Taleb

Types of External Memory Magnetic Disk RAID Removable Optical CD-ROM CD-Recordable (CD-R) CD-R/W DVD Magnetic Tape

Magnetic Disk A disk is a circular platter constructed of nonmagnetic material, called the substrate, coated with a magnetizable material. Substrate used to be aluminium. Now glass. Improved surface uniformity (Increases reliability). Reduction in surface defects, so reduced read/write errors. Lower flight heights. Better stiffness. Better shock/damage resistance.

Read and Write Mechanisms Recording & retrieval done via conductive coil called a head. May be single read/write head or separate ones (two). 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 and Write Mechanisms 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 & speed).

Data Organization and Formatting Concentric rings or tracks. track has same width as the head. Gaps between tracks to prevent or minimize errors. Reduce gap to increase capacity. Same number of bits per track. (variable packing density). Constant angular velocity (CAV). Tracks divided into sectors. Minimum block size is one sector. May have more than one sector per block.

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 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.

Comparison of Disk Layout Methods

Winchester Disk Format

Physical Characteristics

1. 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. 2. Removable or Non-Removable Removable disk Can be removed from drive and replaced with another disk. Provides unlimited storage capacity. Easy data transfer between systems. (FD & ZIP cartridge) Non-removable disk Permanently mounted in the drive. (HD)

3. Single-Double sided 4. Single-Multiple platters Double Sided The magnetizable coating is applied to both sides of the platter. Single sided disk Less expensive. 4. Single-Multiple platters 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).

Tracks and Cylinders Part of one cylinder

5. Head Mechanism Fixed gap Read-write head has been positioned a fixed distance above the platter, allowing an air gap. Contact Physical contact with the medium during a read or write. Floppy disk, which is small, flexible platter and the least expensive type of disk. Winchester heads They are designed to operate closer to the disk’s surface than conventional rigid disk heads (greater data density). The air pressure generated by a spinning disk is enough to make the foil rise above the surface. Use narrower heads that operate closer to the platters.

Disk Performance Parameters Seek time: On a movable-head system, the time it takes to position the head at the track. ( in H.D < 10 ms) Rotational delay (rotational latency): The time it takes for the beginning of the sector to reach the head. The access time: the time it takes to get into position to read or write (seek time and + rotational delay). The transfer time: the time required for the transfer data.

Cont. ** The transfer time to or from the disk depends on the rotation speed of the disk in the following: ** The total average access time can be expressed as:

RAID “It is a set of physical disks drives viewed as single logical drive by operating system”. Redundant Array of Independent Disks The RAID scheme consists of seven levels, zero through six. Not a hierarchy 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 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.

RAID 1 Mirrored Disks (redundancy by expedient of duplicating all data) Data is striped across disks. 2 copies of each stripe on separate disks. Read from either, but Write to both. Recovery is simple. Swap faulty disk & re-mirror. No down time. Expensive (require twice the disk space of the logical disk).

RAID 2 Disks are synchronized (head in same position at a given time). 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. An effective choice when many disk errors occur. Expensive, and not used now.

RAID 3 Similar to RAID 2 (Only one redundant disk, no matter how large). Instead of an error-correcting code, a simple parity bit is computed for the set of individual bits in the same position. Data on failed drive can be reconstructed from surviving data and parity information. In the event of a disk failure, all of the data are still available in what is referred to as reduced mode using the XOR calculation. Very high transfer rates.

RAID 4 Use independent access technique. (Each disks 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 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.

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.

Summary

Solid State Drives (SSD) “It is a memory device made with solid state components that can be used as a replacement to a hard disk drive”. The SSDs now on the market and coming on line use a type of semiconductor memory referred to as flash memory. Two distinctive types of flash memory, NOR and NAND. In NOR flash memory, the basic unit of access is a bit. For NAND flash memory, the basic unit is 16 or 32 bits. SSD Advantages over HDD * High-performance input/output operations per second (IOPS) * Durability. * Lower power consumption. * Longer lifespan. * Quieter and cooler running capabilities. * Lower access times and latency rates.

Optical Memory Originally for audio. 650Mbytes giving over 70 minutes audio. Polycarbonate coated with highly reflective coat (aluminium). Data stored as pits, and Land is located between pits. Read by reflecting laser. Constant angular velocity (CAV), then for increasing capacity Constant linear velocity (CLV) is used.

CD-ROM Block Format User data. The beginning of a block. - Additional user data in mode 2. - Mode 1:288-byte error correcting code. The beginning of a block. 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.

CD-ROM Vs DVD-ROM

Optical Memory Characteristics

Magnetic Tape Serial access as magnetic disk. Slow and very cheap. Backup and archive. Linear Tape-Open (LTO) Tape Drives. Developed late 1990s. Open source alternative to proprietary tape systems.

LTO Tape Drives

H.W (4) 6.3 6.4 6.8 Deadline: Sunday, 02.04.2016

Quiz Main memory = 512 KB Block size = 16 word 1 Word = 1 B Cache size= 4 KB 1. Find the tag, word and line for 7B53E4 Address using Direct-mapping. 2. Find the tag, word and line for 7B53E4 Address using 4-way set associative mapping.

Solution Block size = 16 word = 2^4 ......... W = 4 bits. Main memory = 512 KB .......... Tag + (line/set) = 19 bits. # of lines = 4 K/16 = 2^8 ............ Line = 8 bits. # of lines/set = 2^8/4 = 2^6 .......... Set = 6 bits. Address = 111 1011 0101 0011 1110 0100 Direct-mapping. W = 4 / line = 3E / Tag = 7B5 4-way set associative mapping. W = 4 / line = 3E / Tag = 1ED4 W = 4 Line = 8 Tag = 11 W = 4 Set = 6 Tag = 13

Thank you