Storage: SSD vs HDD, File Systems
Storage is a critical layer in the memory hierarchy, providing persistent storage for data even when power is off.
This article explores Hard Disk Drives (HDDs) vs Solid-State Drives (SSDs), key performance trade-offs, and the role of file systems.
Designed for system design interviews and engineering awareness.
1. Storage in the Memory Hierarchy
Storage devices are slower than CPU caches or RAM but much larger and cheaper. They are ideal for long-term storage of programs, data, and logs.
- Registers / Cache → Nanoseconds, very small.
- RAM (DRAM) → Tens of nanoseconds, volatile.
- Storage (SSD/HDD) → Microseconds to milliseconds, persistent.
- Archival (tapes, cloud cold storage) → Seconds to minutes, very large.
2. HDD vs SSD: Core Comparison
Hard Disk Drives (HDDs)
- Technology: Spinning magnetic platters + mechanical read/write heads.
- Latency: ~5–10 ms (seek + rotational delay).
- Speed: 80–160 MB/s typical.
- Capacity: High (500 GB–20+ TB).
- Cost: $0.02–$0.05/GB.
- Durability: Vulnerable to shock, vibration, and wear.
- Power Consumption: Higher (spinning + moving parts).
- Use Cases: Bulk storage, backups, archival.
Solid-State Drives (SSDs)
- Technology: NAND flash memory, no moving parts.
- Latency: ~50–200 µs (orders of magnitude faster than HDDs).
- Speed: 500 MB/s (SATA SSDs) → 7 GB/s (PCIe NVMe).
- Capacity: Commonly 128 GB–8 TB (larger enterprise SSDs exist).
- Cost: $0.08–$0.20/GB.
- Durability: Resistant to shock, but limited by write endurance.
- Power Consumption: Lower.
- Use Cases: OS drives, databases, gaming, high-performance workloads.
Latency Comparison
| Device Type | Latency |
|---|---|
| Registers | < 1 ns |
| Cache (L1) | ~1 ns |
| RAM (DRAM) | 50–100 ns |
| SSD (NVMe) | 50–200 µs |
| HDD | 5–10 ms |
3. SSD NAND Flash Types & Endurance
SSDs store data in NAND cells, which wear out after many writes. The type of NAND affects performance, cost, and endurance:
- SLC (Single-Level Cell) → 1 bit/cell. Fastest, most durable (~100k writes), expensive.
- MLC (Multi-Level Cell) → 2 bits/cell. Balanced performance, ~10k writes.
- TLC (Triple-Level Cell) → 3 bits/cell. Common in consumer SSDs, cheaper, ~3k writes.
- QLC (Quad-Level Cell) → 4 bits/cell. Highest density, cheapest, lowest endurance (~1k writes).
SSD Endurance Metrics
- TBW (Terabytes Written) → How much data can be written over lifetime.
- DWPD (Drive Writes Per Day) → How many times the drive’s full capacity can be written per day.
Interview Tip: Be ready to explain why enterprise databases prefer MLC/SLC SSDs despite higher cost (endurance).
4. SSD Variants
- SATA SSDs → Limited by SATA interface (max ~600 MB/s).
- NVMe SSDs (PCIe) → Leverage PCIe lanes, speeds up to 7 GB/s.
- M.2 / U.2 form factors → Compact connectors used in laptops and servers.
- Enterprise SSDs → Larger capacity, higher endurance (often with power-loss protection).
5. File Systems
A file system organizes how data is stored, named, and retrieved. It sits between the OS and hardware.
Common File Systems
FAT32
- Pros: Widely compatible.
- Cons: 4 GB file size limit, no permissions.
NTFS (Windows)
- Pros: Large files, journaling, encryption, permissions.
- Cons: Limited cross-platform support.
ext4 (Linux)
- Pros: Journaling, large files, crash recovery.
- Cons: Linux-specific.
APFS (Apple)
- Pros: Optimized for SSDs, snapshots, encryption.
- Cons: Apple ecosystem only.
ZFS
- Pros: Data integrity, snapshots, RAID-like features, deduplication.
- Cons: Complex setup, high memory usage.
6. File Systems & SSD Optimization
File systems influence SSD lifetime and performance:
- TRIM/Discard → Informs SSD which blocks are free, avoiding unnecessary writes.
- Wear Leveling → Evenly distributes writes across cells.
- Journaling FS (NTFS, ext4) → Protects against crashes, but adds write amplification.
Interview Tip: Be able to explain why FAT32 on SSD is inefficient (no TRIM support).
7. Real-World Context
Interview Relevance
- HDD vs SSD latency and throughput.
- NAND types and endurance.
- File system features (journaling, TRIM).
Engineering Practice
- Use SSDs for databases, logs, and random-access workloads.
- HDDs still win for cold storage and cost-sensitive bulk data.
Modern Trends
- NVMe SSDs dominate performance markets.
- ZNS (Zoned Namespace SSDs) optimize write patterns.
- Intel Optane (3D XPoint) offered ultra-low latency (now discontinued, but concept lives in NVM research).
8. Further Reading
- Computer Organization and Design — Patterson & Hennessy
- Operating System Concepts — Silberschatz, Galvin, Gagne
- Intel & Samsung SSD whitepapers
- Research on file systems for SSD optimization