CS240 Disk Operation & Scheduling Vocabulary

Vocabulary flashcards covering key terms from CS240 lecture on magnetic hard disks, SSDs, performance metrics, and disk scheduling algorithms.

Magnetic Hard Disk Drive (HDD)

Mechanical storage device using rotating platters coated with magnetic material for long-term, high-capacity, low-cost data storage.

Platter

Circular disk inside an HDD whose surfaces store data in concentric magnetic tracks.

Track

Concentric ring of magnetic storage on a platter where data sectors are laid out.

Sector

Smallest addressable block on a disk (traditionally 512 B, now often 4 KB) containing preamble, ID, data, ECC, and gaps.

Cluster

File-system unit consisting of one or more contiguous sectors; minimum space a file can occupy on disk.

Actuator

Electromechanical component that moves the read/write heads across the disk tracks.

Read/Write Head

Slider-mounted element that senses or alters magnetic polarity on a platter to read or write data.

Zoned Bit Recording (ZBR)

Technique that stores more sectors on outer tracks than inner ones, boosting capacity and outer-track transfer rates.

Rotational Velocity

Constant spinning speed of platters; with ZBR, outer tracks pass under the head faster, giving higher data rates.

Seek Time

Time for the actuator to move the head to the desired track; dominant component of disk access latency.

Average Latency

Mean delay to reach a sector, including half a rotation on average; typically 4–9 ms for HDDs.

Data Rate

Bandwidth between drive and system, limited by interface (e.g., SATA, SAS, NVMe) and internal media speed.

Capacity

Total amount of user-addressable data a drive can reliably store.

Mean Time To Failure (MTTF)

Statistical measure of drive reliability; HDD electronics often give 4-7 year expected lifetime.

Native Command Queuing (NCQ)

SATA feature letting the drive reorder queued requests internally to minimise seeks.

Shortest Seek Time First (SSTF)

Scheduling algorithm that services the request closest to the current head position, potentially starving outer/inner tracks.

SCAN Algorithm

“Elevator” scheduling that moves the head in one direction servicing requests until the end track, then reverses.

C-SCAN Algorithm

Like SCAN, but after reaching one end the head quickly returns to the start without servicing, giving uniform wait times.

LOOK Algorithm

Variant of SCAN that turns around at the last outstanding request instead of going to the physical end track.

C-LOOK Algorithm

Variant of C-SCAN that flys back after the last request in a direction, avoiding travel to unused end tracks.

Solid State Drive (SSD)

Non-mechanical storage using NAND flash; offers 20–40× faster access, higher cost, less capacity, and finite write endurance.

Floating Gate Transistor

Flash memory cell technology that traps electrons to store non-volatile bits in SSDs.

Serial ATA 3 (SATA 3)

Common consumer HDD/SSD interface supplying up to 6 Gbit/s serial bandwidth.

Serial Attached SCSI-5 (SAS-5)

Enterprise disk interface offering roughly 45 Gbit/s throughput.

NVMe

High-performance PCIe-based protocol for SSDs, delivering greater parallelism and data rates than SATA/SAS.