CMR (Conventional Magnetic Recording) and SMR (Shingled Magnetic Recording) are competing HDD technologies with distinct architectures impacting performance and use cases.
Core Technology Difference
CMR writes data on non-overlapping, parallel tracks. Each track has full width for the read/write head, enabling simultaneous access and faster random writes.
SMR overlaps tracks like shingles on a roof, increasing density. While reading is similar to CMR, writing new data often requires reading, modifying, and rewriting entire overlapping track groups (zones), significantly slowing down random writes.
Key Differences Summary
- Density & Capacity: SMR wins, offering higher capacities per platter.
- Sustained Sequential Write Speed: Comparable performance when writing sequentially within freed zones.
- Random Write Speed: CMR is significantly faster; SMR suffers due to zone rewriting.
- Write Latency & Consistency: CMR offers lower, predictable latency; SMR latency can spike dramatically during zone rewrites.
- Workload Handling: SMR struggles with write-heavy workloads (e.g., databases, video editing scratch disks, virtual machines). CMR handles diverse workloads well.
- Complexity & Management: SMR requires Drive Managed (DM-SMR) or Host Managed (HM-SMR) firmware. DM-SMR hides complexity internally (simpler for users), while HM-SMR requires specific OS/software support. CMR requires no special handling.
- RAID Performance & Reliability: SMR can cause severe performance degradation and potentially timeout failures in RAID arrays due to slow rebuilds and inconsistent response times. CMR is recommended for RAID.
Which One Do You Need? (CMR vs SMR)
- Choose CMR for:
- Performance-critical write operations (databases, server applications).
- Mixed or unpredictable workloads (desktop/gaming PCs, general NAS).
- RAID arrays (ZFS, RAID 5/6, etc.) for reliable rebuilds and consistent performance.
- Video editing scratch disks, virtual machines, caching drives.
- Environments where consistent low latency is crucial.
- Consider SMR for:
- Cost-effective, high-capacity archival or backup storage where data is written once or infrequently and read often (e.g., large media libraries, backup targets).
- Consumer external drives primarily used for file storage accessed sequentially.
- Use cases where price per terabyte is the dominant factor, and performance limitations are acceptable.
- Avoid SMR in NAS labeled as such unless explicitly marketed as SMR-optimized for archival, and you understand the limitations.
Recommendation: Opt for CMR unless your use case is strictly large-scale, sequential-write-heavy archival with minimal random writes/rewrites. CMR provides broader compatibility and significantly better performance for active workloads and RAID configurations.
