2.2.1. Hard Drive Media Types and Interfaces
💡 First Principle: Different storage media exist because no single technology optimally serves all workloads. HDDs offer high capacity at low cost; SSDs offer high performance at higher cost; the right choice depends on the workload's read/write pattern and I/O requirements.
Hard Disk Drives (HDDs)
HDDs use spinning magnetic platters. Performance is primarily determined by rotational speed (RPM):
| Speed | Typical Use Case | Notes |
|---|---|---|
| 7,200 RPM | General-purpose, archive, bulk storage | Most cost-effective per GB |
| 10,000 RPM | Database servers, higher-throughput workloads | Good balance of cost and performance |
| 15,000 RPM | High-transaction databases, latency-sensitive apps | Highest HDD performance, most expensive per GB |
HDD limitations: mechanical failure modes (head crashes, bearing wear), latency measured in milliseconds, and sensitivity to physical shock and vibration.
Solid State Drives (SSDs)
SSDs use NAND flash memory with no moving parts. Key characteristics:
- Read-intensive SSDs: Optimized for workloads that mostly read (web servers, CDN caches). Lower write endurance is acceptable because writes are infrequent.
- Write-intensive SSDs: Higher-endurance flash for workloads with heavy writes (databases, log servers, analytics). More expensive but rated for significantly more write cycles.
- Wear factors: SSDs have a finite number of write cycles per cell. TBW (Terabytes Written) is the rated endurance. Monitoring SSD wear is critical—unlike HDDs, SSDs may fail suddenly when wear limit is reached rather than showing gradual degradation.
Hybrid Drives
Hybrid drives combine an HDD with a small SSD cache. Frequently accessed data is cached on the SSD tier, providing near-SSD performance for hot data while maintaining HDD capacity and cost for bulk storage. Less common in modern server deployments as SSD prices have declined.
Storage Interface Comparison
| Interface | Max Speed | Use Case | Notes |
|---|---|---|---|
| SATA | 6 Gbps | General-purpose servers | Most common for consumer/entry server |
| SAS | 12 Gbps (SAS-3) | Enterprise servers | More reliable, supports longer cables, backward-compatible with SATA |
| NVMe (PCIe) | 32+ Gbps | High-performance, latency-sensitive | Dramatically lower latency than SATA |
| eSATA | 6 Gbps | External storage | SATA over external cable |
| USB | Varies (USB 3.x = 5-20 Gbps) | Temporary/portable storage | Not suitable for primary server storage |
| SD | Varies | Embedded systems, OS booting | Used in some server platforms for hypervisor boot |
⚠️ Exam Trap: SAS is backward-compatible with SATA (a SAS controller can use SATA drives), but SATA is not backward-compatible with SAS (a SATA controller cannot use SAS drives). This asymmetry is a common exam distractor.
Reflection Question: A database server handles 80% reads and 20% writes on a 10 TB dataset. Which drive type and interface would you recommend, and why?
