Storage drive form factor

While it is possible to mix use cases, server strategies, and storage technologies within a single cluster, for clarity here we'll focus on uniform deployments.

The most common physical sizes of storage drives in use today are 3.5" LFF and 2.5" Small. You may also occasionally find the 1.8" form factor that never caught on in a big way. Recent and emerging technologies include PCI-e and hot-pluggable U.2 NVMe solid-state devices.

Major server vendors usually offer both LFF and SFF chassis models. LFF models and rotating drives often offer greater TB/RU (Rack Unit) and TB/unit cost. SFF models and rotating drives typically provide more but smaller drives, and the greater number of spindles presents increased parallelism and aggregate performance at the expense of (usually) lesser rack density and cost per unit of storage. The form factor is thus part of your decision process. Clusters designed for latency-sensitive block storage may favor SFF drives, while those optimized for cost, large capacity, and/or object storage lean toward stalwart LFF drives.

Solid State Disk (SSD) storage is rarely available natively in an LFF form factor; SFF drives are the norm. While SFF drives can be fitted into many LFF bays and carriers via inexpensive adapters and interposers, it is more common to select SFF-native server models. That said, server shoppers today face an embarrassment of options; one popular approach is a server chassis that presents 12 or as many as 72 LFF drives for high TB/RU density while providing a pair of internal or rear-panel SFF bays for boot / operating system use. This avoids a reduction of the cluster capacity by having to divert precious front-panel LFF bays from bulk data service.

In it's early years, SSD storage was mainly used for specialized cases due to the very high cost relative to rotating drives, for example, caching, hot databases, and as an option in laptops where battery power is at a premium and physical shocks are all too common.