Understanding Server Storage Types: HDD, SSD, RAID & Network Storage Explained

Classification by Storage Medium

Disks provide persistent storage and are divided into two main categories: mechanical hard disk drives (HDD) and solid‑state drives (SSD).

Mechanical Disk (HDD)

HDDs consist of multiple platters with two‑sided storage. Data is organized into tracks, cylinders, and sectors (typically 512 bytes). Accessing data requires moving the read/write head to the correct track (seek) and waiting for the platter to rotate, making random I/O slower than sequential I/O.

Common techniques to improve HDD performance include:

Caching to eliminate I/O latency

Filesystem layout and write‑ahead logging (WAL)

Distributing workloads across different disks

Using outer tracks for higher throughput

Optimized seek algorithms

Solid‑State Disk (SSD)

SSDs use flash memory with no moving parts, offering faster read/write speeds and lower latency for both sequential and random I/O. However, SSDs still suffer from erase‑before‑write constraints, causing garbage collection overhead that makes random I/O slower than sequential I/O.

Minimum read/write units differ:

HDD: sector, usually 512 bytes

SSD: page, typically 4 KB or 8 KB

Classification by Interface

ATA/IDE

SCSI

SAS (Serial Attached SCSI)

SATA (Serial ATA)

FC (Fibre Channel)

NVMe (Non‑Volatile Memory Express)

Storage Arrays

Multiple disks can be combined into a storage array (RAID) to increase capacity, performance, and reliability. Common RAID levels (RAID 0, 1, 5, 6, etc.) offer different trade‑offs between cost, performance, and availability.

Network‑Attached Storage

Disks can be grouped into a network storage cluster and exposed via protocols such as NFS, SMB/CIFS, or iSCSI. Performance may be affected by network congestion, so client‑side analysis of load and I/O latency is important.

Disks as Block Devices in Linux

In Linux, a disk appears as a block device (e.g.,

/dev/sda

) with a major and minor number. Partitions are represented as

/dev/sda1

,

/dev/sda2

, etc.

Summary

Server storage types fall into three categories: individual disks (HDD or SSD), storage arrays (RAID), and network‑attached storage. Choose based on price, capacity, performance, and stability—SSD for speed, HDD for large cheap capacity, RAID for reliability and throughput, and network storage for shared access.

Next Topic Preview

Disk Performance Inspection

When facing I/O performance issues, consider the following questions:

What is the system’s IOPS and each disk’s IOPS?

What is the throughput of the system and each disk?

What is the disk utilization?

Which application or user is accessing the disk?

Which filesystem or file is being accessed?