14 - Mass Storage

Platter

A rigid, magnetic disk within a hard drive that stores data; it can have one or two "sides" for recording.

Cylinder

The set of tracks that are at the same arm position across all platters in a disk drive.

Track

A circular path on the surface of a disk platter where data is recorded.

Sector (Block)

The smallest unit of data that can be read from or written to a disk, typically 4096 bytes in newer HDDs.

Seek Time

The time it takes for the disk arm to move the heads to the cylinder containing the desired sector.

Rotational Latency

The additional time spent waiting for the disk to rotate the desired sector to the disk head.

FCFS (First-Come, First-Served)

A disk scheduling algorithm that services requests in the order they arrive; it is fair but does not provide the fastest service.

SSTF (Shortest Seek Time First)

A disk scheduling algorithm that selects the request with the minimum seek time from the current head position; may cause starvation.

SCAN (Elevator Algorithm)

An algorithm where the disk arm moves from one end of the disk to the other, servicing requests along the way, then reverses direction.

C-SCAN (Circular SCAN)

A scheduling algorithm where the head moves from one end to the other, then immediately returns to the beginning without servicing requests on the return trip.

LOOK / C-LOOK

Versions of SCAN/C-SCAN where the arm only goes as far as the final request in each direction rather than the full width of the disk.

Low-level Formatting (Physical Formatting)

The process of dividing a disk into sectors that the hardware controller can read and write; occurs before logical formatting.

Partition

A large group of sectors treated by the OS as a separate logical disk.

Bad Block

A sector on a disk that is defective and unusable; the controller or OS must map these out to prevent data loss.

RAID (Redundant Array of Independent Disks)

A technology that provides data redundancy or improved performance by using multiple disk drives together.

Mirroring (RAID 1)

A RAID technique that duplicates data across two or more disks to provide high reliability.

Striping (RAID 0)

A RAID technique that splits data across multiple disks to increase speed, though it does not improve reliability.

Host-attached storage

Storage that is accessed through a local I/O port, such as a standard internal drive or RAID array.

Network-attached storage (NAS)

Storage made available over a network rather than a local connection, often consuming network bandwidth.

WORM (Write Once Read Many)

A type of tertiary storage, like a DVD-R, where data can be recorded once but never changed.