Types of Computer Storage and an Overview of RAID

Computer storage is mainly divided into four categories:

Primary storage : directly connected to the CPU; the CPU continuously reads instructions stored here and executes them as needed. Example: main memory.

Secondary storage : not directly connected to the CPU; it uses I/O channels and cache to transfer data to primary storage. Example: mechanical hard drives and solid‑state drives.

Tertiary storage : large‑scale storage devices that can be inserted or removed from a computer, such as magnetic tape.

Offline storage : storage devices that require manual operation to access, for example optical discs and USB flash drives.

Direct‑Attached Storage (DAS) refers to storage that is directly connected to a computer. Devices like SSDs, HDDs, and optical discs belong to DAS. Generally, secondary and tertiary storage are considered DAS, and offline storage that does not use network transmission also usually falls under DAS.

RAID (Redundant Array of Inexpensive Disks) utilizes virtualization to combine multiple hard disks into one or more logical arrays, aiming to improve performance, provide data redundancy, or both.

In simple terms, RAID merges several physical disks into a single logical disk, so the operating system sees it as one drive. RAID is commonly used on servers, often with identical disks. As disk prices drop and RAID functionality integrates more tightly with motherboards, it has become a popular choice for ordinary users needing large storage capacities, such as video and audio production.

For further study, refer to the comprehensive resources "Server Fundamentals Complete Guide (Ultimate Edition)" and its sister volume "Storage System Fundamentals Complete Guide," which cover storage media technologies, system architectures, protocols, virtualization, backup solutions, and emerging trends.

Additional reading includes articles on multi‑path and iSCSI SAN storage, as well as storage basics and FC SAN introductions.