Retains data even when power is off. Can either be magnetic, solid-state, or optical media.
Terminology
Section titled “Terminology”Transfer Rate
Section titled “Transfer Rate”How fast bits are read or written. Measured in bits per second. Can be theoretical or practical.
Logical Formatting
Section titled “Logical Formatting”Aka. high-level formatting. Prepares a storage device for use by creating file system structures. Done by OS using disk management software.
Hard Disk Drives
Section titled “Hard Disk Drives”Aka. HDD. Magnetic. Cost per GB is low. Theoritcal transfer rates up to 6 GBps. Effective transfer rates up to 1 Gbps. Typical access times 5–15 ms.
Best for sequential access. Mechanical parts can wear out or fail. Sensitive to shocks, vibrations, magnetic fields, and temperature extremes.
Platter
Section titled “Platter”A circular disk coated with magnetic material. Data can be corrupted if exposed to high temperatures or magnetic fields or shocks.
Spindle
Section titled “Spindle”Holds and rotates platters at high speed. Typically 5400–15000 RPM.
Read/Write Heads
Section titled “Read/Write Heads”Float above each platter surface. Magnetize spots to represent bits.
Actuator Arm
Section titled “Actuator Arm”Moves the heads radially to position them over the correct track.
Tracks
Section titled “Tracks”Circular paths on the platter surface where data is stored. Centered around the spindle. Around 50-100 thousand tracks per platter.
Sectors
Section titled “Sectors”Small unit of readable/writeable data. Circular segments within tracks. Each sector contains header information, data, error correction code. Usually data size is 512 bytes or 4096 bytes. 500 to 2000 sectors per track. More sectors on outer tracks due to larger circumference.
Blocks
Section titled “Blocks”Continguous sectors form a block. Data is moved between disk and main memory in blocks. Typical block size is 4 KB.
Low-level formatting
Section titled “Low-level formatting”Aka. hardware formatting. Prepares disk for use by creating sectors. Usually done by manufacturer.
Seek Time
Section titled “Seek Time”Time to move the head to the correct track. Approximately proportional to seek distance. Typical: 3–10 ms.
Rotational Latency
Section titled “Rotational Latency”Time for the desired sector to rotate under the head.
Controller Overhead
Section titled “Controller Overhead”Time taken by the disk controller to process commands. Typically 0.2–0.5 ms.
Transfer Time
Section titled “Transfer Time”Time to read/write the bits once head is in position. Depends on data rate.
Access Time
Section titled “Access Time”Sum of seek time, rotational latency, and controller overhead.
Total access time
Section titled “Total access time”Aka. Total I/O block time. Sum of seek time, rotational latency, controller overhead and transfer time. Typically 5–15 ms for random access.
Average I/O time
Section titled “Average I/O time”Sum of access time, transfer, controller overhead.
Magnetic Tape
Section titled “Magnetic Tape”Used in the early days of computing for secondary storage. A tape coated with magnetic material, is kept in a spool and is wound or rewound past a read–write head. Seek time is in the order of minutes. Transfer time is comparable to HDDs.
Large capacity. Some tapes have built-in compression.
Low access time. Random access is about 1000 times slower than HDDs, and a 100,000 times slower than SSDs.
Mainly used for backup of infrequently used information and to transfer information between systems.
Non-Volatile Memory
Section titled “Non-Volatile Memory”Aka. NVM. No moving parts. Can be built using NAND flash, NOR flash, 3D XPoint, etc. Can work as main memory or secondary storage.
Faster. Slower than DRAM. High thoroughput compared to HDD. More reliable. Optimal for random access.
Cost per GB is higher, compared to HDD. Less capacity.
Includes SSDs, USB drives, flash storage, etc.
Solid State Drives
Section titled “Solid State Drives”Aka. SSD. Physically similar to HDDs. A type of NVM. Built using NAND flash memory (arrays of floating-gate transistors). Data organized into pages (~4 KB). Pages are grouped into blocks (~256 pages).
Extremely low latency (tens of µs). High throughput via parallel memory channels. Instant random access. Shock-resistant and silent. Lower power consumption. High durability.
Limited write cycles (wear leveling required). Write in pages. Erase in blocks. Limited write endurance (mitigated by controller algorithms). Erase operations are much slower and cause wear.
Smallest unit of read/write operation. Cannot erase a single page.
Group of pages. Smallest erasable unit.
Drive Writes Per Day
Section titled “Drive Writes Per Day”Aka. DWPD. Measuring unit of life span. A drive write means writing the whole drive capacity.
Optical Storage
Section titled “Optical Storage”Very low cost per GB. Access time is typically 100–200 ms. High durability, ideal for distribution or archiving.
- Capacities:
- CD: ~700 MB
- DVD: 4.7–8.5 GB
- Blu-ray: 25–50 GB
Uses laser reflection on a spiral track embedded in a polycarbonate disc. Data encoded as pits (non-reflective) and lands (reflective). Track forms a continuous spiral from the disc center outward.