Shingled magnetic recording (SMR) is a storage technology used in some hard disk drives to increase storage density and overall capacity. SMR overlaps (or “shingles”) tracks on a disk platter like roof shingles. This allows more tracks to be crammed onto a platter. However, SMR does come with some notable downsides compared to conventional magnetic recording methods.
Higher write amplification
One key disadvantage of SMR is higher write amplification. Write amplification refers to the amount of data that needs to be rewritten when updating existing data. With SMR, any update to a shingled track requires adjacent tracks to be rewritten as well since the tracks overlap. This amplifies the amount of writing required compared to updating a non-shingled track. The write amplification penalty applies to both overwrite operations and random writes. SMR drives can have a write amplification factor between 1.5x to 8x, depending on the drive and workload. The higher write amplification makes SMR less suitable for write-intensive workloads.
Lower performance for random writes
Due to the write amplification drawback, SMR drives suffer from poor random write performance. Random write performance can be up to 100x slower on SMR compared to conventional magnetic recording. This makes SMR unsuitable for workloads with sustained random write patterns like databases and other transactional applications. The drive must flush and rewrite entire bands of shingled tracks for each small random write, significantly reducing I/O operations per second.
Requirements for larger write buffers
To help mitigate the write amplification issues, SMR drives require larger write buffers/caches to queue up writes. Writes can first be buffered and then flushed to disk sequentially rather than as small random writes. However, the increased memory requirements raise the cost of SMR drives. The larger write caches also increase the amount of data that could potentially be lost in the event of a sudden power outage..
Zone resets hurt performance
SMR drives divide the platters into zones. When a zone fills up, the drive must reset that region by rewriting all the tracks to empty the zone. This zone reset process pauses incoming writes and causes a temporary but significant reduction in write performance until the reset finishes. The impact is similar to a RAID rebuild slowing storage performance.
Drive management overhead
Managing SMR is more complex than standard drives. The SMR drive firmware has to handle write scheduling, cache management, and zone resets. The drive has to track the location of bands of shingled tracks. There is computational overhead to determine optimal data placement and flush order to minimize performance impacts. All this SMR management happens internally on the drive itself. The complexity can lead to firmware bugs or inefficiencies that impair real-world performance and reliability.
Compatibility concerns
Operating systems and other software have to be SMR-aware to handle SMR drives properly. For example, Windows and Linux have added optimizations for SMR management over time. However, older operating systems may not handle SMR well. SMR drives also require modifications to software like databases, file systems, and backup tools to avoid triggering excessive zone resets. Lack of software compatibility can nullify the density benefits of SMR.
RAID rebuild performance penalties
The write amplification challenges of SMR drives have implications for RAID setups. Rebuilding an SMR drive in RAID leads to far more I/O and extra zone resets as all data gets rewritten. This can slow rebuild times significantly compared to conventional drives. Some storage vendors do not recommend SMR drives for RAID environments.
Difficult to mix SMR and non-SMR drives
You cannot easily mix SMR and non-SMR drives in the same RAID array. The rebuild and random write penalties of SMR drives would drag down the overall array performance. This makes transitioning to SMR drives more disruptive, since you generally have to replace all the drives in an array at once.
Reduced reliability
The overlapping write patterns of SMR may reduce long-term reliability. Constantly writing to partially overlapping tracks introduces additional noise between tracks over time. Some SMR drives have higher annualized failure rates compared to conventional magnetic recording designs. However, SMR reliability continues improving with newer generations.
Inability to sanitize instantly
With standard drives you can instantly sanitize deleted content by overwriting it a few times. This is not possible with SMR since overwriting would require rewriting adjacent tracks as well. There is no way to selectively overwrite only desired tracks without touching the neighboring shingled tracks. This makes quick sanitization of deleted content impossible on SMR drives.
Susceptible to environmental vibration
The overlapping shingled tracks make SMR drives more prone to issues from physical environmental vibration. Sustained vibration can cause tracks to become misaligned over time, leading to data errors and drive failure. Vibration resistance has improved with new SMR generations but remains a concern compared to conventional drives.
Limited ability to revise written data
Revising written data on an SMR drive may require rewriting adjacent tracks, akin to having to lift up multiple roof shingles to replace one shingle underneath. By contrast, non-shingled drives make small targeted updates simple. The inability to easily revise data can significantly reduce performance for databases, analytics logs, and other workloads requiring frequent updates.
Potentially misleading SMR marketing
Some hard drive vendors were criticized for selling SMR drives in consumer product lines without proper SMR labeling. This led to accusations of deceiving buyers expecting higher performance. SMR advantages like higher capacity were marketed without equally highlighting the SMR drawbacks. More transparent SMR marketing has improved, but surprises may still occur if drives are not carefully benchmarked.
Difficulties with sequential writes
While marketed as an alternative to low-RPM high-capacity drives for sequential write workloads, SMR drives can struggle with purely sequential write performance as well once zones start to fill up and require resetting. Real-world sequential write throughput may be inconsistent and well below marketed speeds.
| Disadvantage | Summary |
|---|---|
| Higher write amplification | Updating SMR tracks requires rewriting overlapping adjacent tracks, amplifying writes. |
| Lower performance for random writes | Random writes are far slower on SMR drives due to write amplification. |
| Requirements for larger write buffers | SMR needs big caches to buffer writes and reduce amplification. |
| Zone resets hurt performance | Resetting zones by rewriting all tracks causes delays. |
| Drive management overhead | Complex SMR algorithms introduce computational overhead. |
| Compatibility concerns | SMR needs OS, software, and hardware support to avoid issues. |
| RAID rebuild performance penalties | SMR drive rebuilds in RAID are much slower than standard drives. |
| Difficult to mix SMR and non-SMR drives | Cannot easily combine SMR and non-SMR drives in one RAID array. |
| Reduced reliability | Constantly overlapping writes may decrease long-term reliability. |
| Inability to sanitize instantly | Selectively overwriting tracks for instant sanitization is impossible. |
| Susceptible to environmental vibration | Vibration can misalign overlapping tracks over time. |
| Limited ability to revise written data | Updating written data may require adjacent track rewrites. |
| Potentially misleading SMR marketing | SMR downsides may not be adequately disclosed upfront. |
| Difficulties with sequential writes | Zones filling up hamper purely sequential write throughput. |
Conclusion
Shingled magnetic recording certainly provides higher storage density and capacity compared to conventional hard drive recording methods. However, these benefits come at the cost of significant write performance penalties, overhead, and other disadvantages that make SMR a poor fit for many applications. SMR introduces complexities surrounding write amplification, performance inconsistency, vibration resistance, drive compatibility, RAID rebuilds, and more. Careful benchmarking and workload testing is required to determine if SMR drives can still deliver advantages despite their limitations.
SMR technology and algorithms continue improving to mitigate some of the downsides over time. But the underlying write amplification challenges remain inherent to the shingling approach. For applications requiring strong random write performance, low latency, or consistent sequential throughput, SMR is likely a poor fit still today. However, SMR does help squeeze more total capacity from platter drives, at a reasonable cost. The density increases make SMR potentially viable still for certain large-scale sequential write workloads where sheer capacity needs outweigh performance consistency concerns. But the complexities and workload-specific nature of SMR mean diligent testing is required to determine suitability.