ISA343 Chapter #5: Data Storage Technology

SSD vs Magnetic Disk Drives

- SSDs are more portable since no moving parts - SSDs are faster in either random or sequential reads - SSDs are significantly more expensive per GB - Magnetic Disks don't degrade from writing operations, unlike Flash devices like SSDs

Factors that influence the Stored Magnetic Charge

- Strength of the "Write" magnetic field generated by the R/W Head - Mass of the coercible material - Magnetic properties of coercible material - Loss of charge due to magnetic leakage, magnetic decay, and loss of coercible material

Memory Packaging examples

1. 30-Pin SIMM 2. 72-Pin SIMM 3. DDR DIMM 4. DDR2 DIMM

Methods of storing Electrical Signals

1. Batteries 2. Capacitors 3. Mechanical Switches 4. Transistor-Based Switches

Examples of Optical Disks

1. CD-ROM 2. CD-R 3. CD-RW 4. DVD-ROM 5. DVD+/-R 6. DVD+/-RW 7. BD-R 8. BD-RE 9. Magneto-optical 10. HVD

Magnetic Disk Performance Factors

1. Head-to-Head Switching Time 2. Track-to-Track seek time 3. Rotational Delay

Examples of Secondary Storage Devices

1. Magnetic Tape 2. Magnetic Disk 3. Optical Disk 4. Flash RAM 5. Solid-State Drive (SSD)

Techniques to reduce Wait State between CPU and RAM

1. Read-ahead memory access 2. Synchronous Read operations 3. On-chip caches Devices often use all 3 methods.

Three major access methods of Storage

1. Serial Access 2. Random Access 3. Parallel Access Most devices use 2 or all 3 methods.

Characteristics of a Storage Device

1. Speed 2. Volatility 3. Access Method 4. Portability 5. Capacity

Examples of Primary Storage Devices

1. Static RAM (SRAM) 2. Dynamic RAM (DRAM) 3. Nonvolatile Memory

Uninterruptible Power Supply (UPS)

A battery power source that provides electricity to components in case of power failure.

Programmable ROM (PROM)

A blank circuit is created that is then written to once and only once. It cannot be changed once something writes to it.

Storage Medium

A device or substance in which data is stored. Examples: - Storage circuitry of a RAM stick - Metallic surface of a magnetic disk - Reflective surface of a optical disk

Read/Write (R/W) Mechanism

A device used to access (Read) and store (Write) data values onto the storage medium. Examples: - Access circuitry of a RAM stick - Magnetic R/W head in magnetic disk - Laser & Photo Detector in an optical disk

Sector (Magnetic Disk)

A fraction of a track holding a certain amount of Bytes. Before 2009 the standard of a sector was 512 bytes. After 2009, it was changed to 4096 bytes.

Average Access Time

A more specific measure of Access Time for devices that have variable access times. This measure is the sum of the Average Access Delay plus the Sequential Access Time

Magnetic Disk

A series of rotating platters covered in a coercible material. Several parts: - Platter - Spindle - Track - Cylinder - Sector

Dye Based

A stronger powered laser than the Read Laser burns dark spots into the disk. These dark spots give low reflectivity and represent a 0. All other spots that are not burned at a 1. This is a slow process since the laser makes multiple passes. Disks come blank and all possible spaces represent 1. This changes when you burn it. It cannot be undone or changed after the first burn. CD-R is a format that uses this tech.

Optical Disk Writing Methods

AKA Bit Encoding Methods 1. Pits and Lands 2. Dye-based 3. Phase-change

Random Access

AKA Direct Access Any data on the storage medium can be accessed in the same amount of time, regardless of the location. Random Access is not parallel. Only one location is accessed at a time. Disks are an example of Random Access

On-Chip Caches

AKA Enhanced DRAM (EDRAM) Used in combination with Read-ahead. This is the area where the Read-ahead function stores the data it's retrieved for the next instruction. This waiting area is comprised of SRAM. The actual storage that's being used in the currently running instruction is the DRAM and not part of the on-chip cache.

Areal Density

AKA Recording Density or Bit Density The surface area required to store 1 bit measured in bits per area. Typical units are bits, bytes, or tracks per inch. Halving the set storage area for 1 bit will increase the Areal Density, but will intensely increase degradation to the coercible material because it reduces in mass of the material by a factor of 4.

Read-Only Memory (ROM) Technologies

All are non-volatile memory devices. These are all archaic technologies. 1. Read-Only Memory (ROM) 2. Programmable ROM (PROM) 3. Erasable PROM (EPROM) 4. Electronically-Erasable PROM (EEPROM)

Memory Storage Hierarchy

All computer systems contain multiple types of storage devices that function in certain ways. Each device has a certain purpose and function it fulfills. Each device has it's own cost Vs performance trade-off.

Non-Volatile RAM (NVRAM)

Any RAM device that can hold content without continuous power. Flash RAM is the first and most commonly used NVRAM.

Serial Access

Archaic method of storage access. It is slow and inefficient. All storage locations are organized sequentially. In order to access a specific location, the device needs to skip over every storage location before your specific location. Magnetic Tapes are Serial Access only. You need to rotate the tape until the spot you want is found.

Cost Rule of Storage

As you increase a certain characteristic of a storage device, other characteristics will decrease/change. If you don't want the other characteristics to change, the $ cost of the device will increase.

Erasable PROM (EPROM)

Black circuit that is written to by a device. Circuit can be erased by exposing to ultra-violet light and written to again.

Read-Only Memory (ROM)

Circuitry stores the state of electrical signals. It cannot be written to and cannot be changed. The circuit literally is designed to hold the specific data.

Cloud Based Storage

Cloud provider managers large amounts of storage hardware in a remote location shared by many users. This gives economies of scale. Data gets stored to multiple remote locations to prevent loss High speed network increases speed of data movement Client side is synced to cloud copies regularly. Software manages data movement between server and client and protect the data.

Data Transfer Rate

Data Transfer Rate is the amount of data transferred in a set unit of time. Normally this is a single second. 100Mb in 1 second. 100Gb in 1 second.

Electrical Signal Storage

Data is represented as electrical signals in the actual hardware. To store data, we need to either: - Store electrical signals directly - Use electrical signals to generate something that can be stored.

Sector (Transfer Unit)

Describes the amount of data transferred in one R/W operation. Used ONLY in the context of optical or magnetic disks. Usually is 512 Bytes (pre 2009) or 4096 (post 2009)

Block

Describes the amount of data transferred in one R/W operation. Used in the context of any storage device.

Defragmentation

Disk reorganization process that takes scattered sectors put them next to each other in a sequential way. This achieves the most efficient organization: - Sequential sectors in a track - Sequential tracks in a cylinder - Sequential cylinders

Flip-Flop Switches

Electrical Switch that remembers it's last position as long as power flows through it. Flip-flops are the basic component of SRAM and CPU registers.

Transistor-Based Switches

Faster than Mechanical Switches. Like mechanical switches but smaller. If electricity is cut off, the switches dump their current state. This makes it volatile.

Capacitors

Faster than batteries. Need to recharge once energy is expended. Can dispense and charge significantly faster than a battery. This limits the capacity.

Mechanical Switches

Faster than capacitors and batteries. Non-volatile jumpers that stay in place once flipped. Requires energy to flip.

Maximum Data Transfer Rate

Fastest rate at which a disk can deliver data to other system components. Assumes that disk is not fragmented, disregards the H2H and T2T seek times as irrelevant, and only considers the Access Delay.

Media Integrity

Friction from the R/W process can wear away the coercible material coating from tapes and floppy disks. Time, physical stress, heat, and humidity can weaken the bond between the coercible coating and the substrate. Mainly seen in tapes.

Device electronics and Mechanics limitation to Magnetic Storage

If you increase the speed or capacity of the magnetic medium, you'll also need to improve the speed and position placement of the R/W heads. They're fundamentally tied together.

Electronically-erasable PROM (EEPROM)

Instead of UV light, electrical signals instruct the circuit to reset it's storage. Therefore, a section of the circuit exists purely to erase the rest of the circuit.

Device Controller

Interface device that connects the storage device to the system bus. Requests and responses are passed along this connection from the drive to the system.

Secondary Storage

Larger quantity, slower storage area for data. This storage is often non-volatile and low cost.

Optical Disk Read Operation

Laser bounces off surface of disk. Photodetector detects a high or low amount of reflected light. High reflectivity = 1 Low reflectivity = 0

Magnetic Decay

Loss of magnetic charge strength over time. This is unstoppable and will depend on the mass and chemical material of the coercible material.

Magnetic Storage Principles

Magnetic Storage devices take electrical signals, which represent bits, and translates them into variations in the magnetic field of a specific location on a magnetic storage medium. The storage medium is typically something magnetic or anything that is coercible.

Variable Sector Density

Manufacturers will divide the tracks of a platter into zones in a Magnetic Disk. This increases capacity per platter and the overall Disk. The number of sectors will vary per zone.

Coercible Material

Materials that will accept and hold a magnetic charge. Most often is a coercible coating onto a material. Examples: - High purity metals on disk platters - Iron-oxide or chrome oxide on magnetic tapes

Phase Change

Much like PRAM, the disc is coated in a substance that can either be Crystalline or Amorphous. Crystalline state is highly reflective representing 1. Amorphous state is not reflective representing 0. This can be re-written multiple times. Write times are slow since multiple passes of the laser is required.

Magnetoresistive RAM (MRAM)

Newer Non-volatile RAM Tech Stores bits onto two magnetic elements: - One element that has a fixed magnetic polarity - One element that varies it's polarity when a bit is written The electrical resistance between the two magnetic elements determines the value of the bit that is stored: - Low Resistance = 0 - High Resistance = 1 Speed like SRAM, Capacity like DRAM, Writes are not destructive like Flash RAM so increased lifetime.

Phase-Change Memory (PRAM or PCRAM)

Newer non-volatile RAM tech. Germanium, antimony, or tellurium is used to create a switch. That switch changes from amorphous or crystalline states when heated to reflect a 1 or 0 value. An amorphous state will have poor reflectivity and high electrical resistance. A Crystalline state will have high reflectivity and low electrical resistance. Lower storage density and slower read time than Flash RAM. Faster Write time than Flash with a slower degradation rate than Flash.

Optical Disk Vs Magnetic Disk

Optical Disk: - Removable Platters - Single platter normally - Slower RPMs - Writing is slower than reading - Variation in reflectivity is how bits are stored - R/W head uses lasers and photodetectors to R/W Magnetic Disk - Non-removable platters - Multiple platters normally - Faster RPMS - Read/Write times are pretty similar - Direction of magnetic charge is how bits are stored - R/W Head uses magnetic current generator to R/W

Pits and Lands

Pits are concave areas that scatter light and reduce reflectivity. Lands are flat areas that reflect light and increase reflectivity. Read-Only CDs and DVDs are formats that use this tech.

Portability Characteristic of Storage

Portability comes in two forms: 1. The device and the medium inside is portable (USB Flash Drives, HotSwap Hard Drives) 2. The medium can be removed and placed in a different device that can read it. (CDs, DVDs)

Random Access Memory (RAM)

Primary Storage technology with the following characteristics: - Bits are stored in transistors and/or capcitors - Has Semiconductor chip(s) - Read and Write access time is equal - Combination of random and parallel access methods Two basic types: - Static RAM (SRAM) - Dynamic RAM (DRAM)

Primary Vs Secondary Characteristic Differences

Primary is: - Fast - Volatile - Parallel Access - Non-portable - Expensive Secondary is: - Slow - Non-volatile - Various in access methods - Potentially portable - Less Expensive

Flash RAM

RAM storage area is made persistent with a specific transistor type that stays "open or closed" without electricity. Cost and density is comparable to DRAM. Transfer rates max out around 560MBs compared to DRAM's 20GBs Flash devices will eventually fail as every single write operation degrades the device.

Sustained Data Transfer Rate

Rate that is based on the assumed "typical distrubution" of data. Unlike the Maximum Data Transfer Rate, this rate considers the impact of disk fragmentation.

Solid State Drive (SSD)

Secondary storage device that uses Flash Memory in a way that mimics Magnetic Disks. Insanely fast at 63GBs

Cylinder

Set of tracks, across each platter, that are at the same position from the edge. Platter A's Track 2 is 3mm from the edge. Platter B's Track 45 is also 3mm from the edge. These tracks would make a cylinder together.

Parallel Access

Similar to Random Access, but multiple locations can be access simultaneously within the same amount of time. An array of Disks functions with Parallel Access.

Speed Characteristic of Storage

Speed covers several questions: 1. Locate Data Speed? 2. Transfer Data Speed? 3. Maximum simultaneous R/W requests? 4. Maximum amount of data being R/W at the same time? Speed has several distinct measures that answer these questions.

Wait State

Speed mismatch between the CPU and the Primary Storage. Essentially: The pace of the CPU is not in sync with the Primary Storage. You need to minimize wait states to increases the speed of the information between moving from the Primary Storage to the CPU processing it.

Batteries

Stores energy. Poorly suited for rapid storage/retrieval Need to recharge once energy is expended.

Magnetic Reading Operation

The R/W Head locates the area where the data is stored. The stored charge is activated and the data is transmitted back to the R/W Head.

Magnetic Writing Operation

The R/W head generates a magnetic field with the same polarity as the electrical signals flowing through the head. That magnetic field stores a magnetic charge onto the magnetic storage medium. The polarity direction of the charge (Left to Right or Right to Left) will represent a 0 or 1 when the magnetic charge is stored on the medium.

Read-ahead Memory Access

The RAM device gets the next memory location for the next instruction while it delivers the current instruction's request.

Real Sustained Data Transfer Rate

The actual transfer rate of a drive which lays between the Maximum Data Transfer Rate and the Sustained Data Transfer Rate.

Primary Storage

The area where instructions and data for immediate/frequent use in the CPU is stored. Primary storage focuses on accessing information very fast. It does not have high storage capacity as this would impact speed. Wait states are one of the factors that impacts the retrieval speed of Primary Storage.

Magnetic Leakage

The cancellation of magnetic charge in adjacent areas of opposite polarity. Causes a rapid loss of charge and destruction of data. Worsens in magnetic storage that uses 3 dimensions like magnetic tape.

Spindle

The center mounting of a Magnetic Disk that is attached to a motor. Multiple platters are attached to rotate on the Spindle.

Read Threshold

The minimum amount of energy required to generate a detectable current flow in R/W Head. If a stored magnetic charge on the magnetic medium falls below that threshold, the data can't be read.

Track

The portion of a disk that passes under the R/W head in 1 rotation. Platters have many tracks that are organized into 1 bit wide rings from the edge of the platter to the center.

Platter

The round substrate that coercible material is coated onto in a Magnetic Disk. Platters have multiple tracks that are read by the R/W Head. Platters rotate on a spindle past a R/W Head.

Access Time

The time required to complete a single R/W operation. Amount of Seconds req to: - Accept the R/W Command - Find the location of the data - Transfer the data from the location Access time is constant for some storage devices, like RAM, and variable for others, like disks. Access Time does not answer the questions of how much data is transferred in a R/W or if Access Time increases with repeated R/Ws.

Average Access Delay

Time required to move between two sectors separated by an average number of recording surfaces, tracks and sectors. Avg Access Delay = Head-to-Head Switching Time + Track-to-Track Seek Time + Rotational Delay Time

Track-to-Track Seek Time (T2T)

Time required to move the R/W head from their current position to the next Track. Variable speed depending on the size of the platter and number of tracks. Relatively slow since it's a mechanical movement. Measured in an average.

Sequential Access Time

Time to read two adjacent sectors on the same track and the same recording surface. Depends on rotation speed.

Rotational Delay

Time waiting for a desired sector to rotate beneath the R/W Head. Relatively slow since it's a mechanical movement. Higher rotation per minute (RPM) will decrease this delay. Measured in an average based on 1/2 rotations.

Magnetic Tape

Type of Magnetic Storage Device Coercible material is coated onto a plastic ribbon and wound onto a spool. A motor turns the spool and pulls it past a R/W head to read the tape. VHS tapes are an example of this.

Static RAM (SRAM)

Uses Flip-Flop circuits and is very fast. Volatile. Expensive. Is often the "waiting area" for Read Ahead functions to store data for consecutive R/W operations. On RAM this is the on-chip cache.

Optical Disk Storage

Uses a platter with optical properties instead of Magnetic. Data is recorded as variations in reflectivity.

Dynamic RAM (DRAM)

Uses transistors and capacitors. Volatile. Limited in speed by the rate at which their capacitors recharge. Slower and less expensive than SRAM. Is the "Real" space of the RAM stick. eg: 8GB of RAM is the Gb of DRAM.

Head-to-Head Switching Time (H2H)

Usually, only one circuit is managing which R/W head is currently active in a Magnetic Disk. That circuit needs to rotate through each R/W Head before it can activate the next head. R/W Head #1 to #5 will require 4 switching operations. This is under 1 nanosecond in modern Mag Disks

Synchronous Read Operation

Variation of the read-ahead function called Synchronous DRAM (SDRAM) RAM attempts to be synced to the clock of the CPU or the Bus. This keeps it from being out of sync and creating Wait State. Can also run on a fraction of the clock's state. Double Data Rate (DDR) SDRAM works on every 0.5 second tick.

Volatile vs Non-volatile Storage

Volatile storage dumps it's stored data when the power is cut. Non-volatile storage keeps the data when the power is cut. Temporary VS Permanent

Fragmentation

When data contents are created and deleted over time, sectors of a single file tend to scatter across the disk in random locations. Substantially reduces the R/W performance of the disk since it has to look longer for the same file.

Non-volatile Storage Lifetime

While non-volatile storage is essentially permanent, it is limited by the lifetime of the device. Flash RAM often degrades with more R/W operations. Mag Disks degrade with constant use, but will hold information for decades if powered off. RAM, if powered using a UPS, will not dump it's contents and act as Non-volatile storage indefinitely assuming power is never lost.

Memory Packaging Evolution

[Pre 1980s]: Dual-Inline Package (DIP) [Late 1980s]: Single In-Line Memory Modules (SIMMs) [2000+]: Dual In-Line Memory Modules (DIMMS)