Flash storage, also known as solid state drives (SSDs), have become a popular data storage technology in recent years due to their fast read and write speeds compared to traditional hard disk drives (HDDs). However, flash storage does come with some disadvantages that should be considered when deciding whether it is the right choice for your storage needs.
Limited number of write cycles
One of the main disadvantages of flash storage is that it has a limited number of write cycles. This refers to the number of times data can be written to the drive before it can no longer reliably store data. Flash storage cells degrade each time they are written to, meaning they have a finite lifespan.
Most consumer-grade flash storage drives are rated for anywhere from 1,000 to 10,000 write cycles per cell. While this may sound like a lot, it’s not as high as the write endurance offered by HDDs, which can typically withstand hundreds of thousands to millions of write cycles before failure. If you frequently write and rewrite data to the drive, flash storage will wear out more quickly than HDDs.
Potential for data loss
The limited number of write cycles poses another risk – the potential for catastrophic data loss. Once flash storage cells degrade past a certain point, they may no longer be able to reliably retain data. If enough cells fail simultaneously, the result could be irrecoverable data loss.
To mitigate this risk, flash storage controllers perform “wear leveling,” which means spreading out write operations across all the cells to avoid overusing any single cell. However, once a drive is nearing the end of its write cycle lifespan, the risk of unrecoverable failure starts to rise.
Higher cost per gigabyte
Flash storage typically costs more per gigabyte compared to hard disk drives. As of 2023, a 1TB HDD costs around $40 while a 1TB SSD typically costs around $80-$100. The price discrepancy is narrowing as flash storage prices fall, but HDDs remain significantly cheaper per gigabyte.
There are a few reasons for the cost difference. Flash storage fabrication requires higher-quality materials like silicon wafers, and the manufacturing process is more complex. In addition, there is a limited number of flash memory fabrication plants. As demand rises, prices remain elevated compared to the abundant supply of HDDs.
Slower write speeds at high capacity
Although flash storage offers very fast read speeds, write speeds are more variable and tend to decrease as drive capacity increases. This effect occurs because higher capacity drives often have additional memory chips that are less directly connected to the controller.
For lower capacity SSDs, write speeds can approach read speeds. But for high capacity models, write speeds may be 3-4x slower than read speeds. If your workload involves heavy write activity as opposed to mostly reads, you may want to choose a lower capacity drive.
Susceptibility to environmental threats
Flash storage cells rely on a very small electrical charge to represent data values of 0 or 1. This makes them susceptible to data loss from power failures or hardware malfunctions. Sudden power loss can erase recently written data that has not yet been committed to long-term storage. High heat can reduce data retention time.
Hard disk drives are not immune to environmental threats either. But the mechanical nature of their magnetic storage tends to make them a little more resilient. With HDDs, even if the heads or motors are damaged, there is still a good chance of recovering the magnetic data.
Faster wear from erase/program cycles
In addition to wear from writing new data, flash storage cells also degrade from erase cycles. Before cells can be rewritten, they first need to be erased, usually in large blocks. This erase function contributes significantly to cell wear.
The erase effect happens because an erase sets a cell to its maximum voltage state. Bringing a cell to full voltage stresses it more than the usual small voltage bumps from writes. Vendors specify flash storage endurance in terms of drive writes per day that include both writes and erases.
Performance impact from garbage collection
To reuse cells for writing, SSD controllers run a garbage collection process in the background that erases unused blocks with invalid stale data. This process can sometimes negatively impact performance.
When garbage collection kicks in, the SSD may pause active processes writing data so that it can first erase unused blocks to free up space. This can lead to stuttering performance where data write speeds drop significantly for short periods. With HDDs, there is no background garbage collection that impacts performance.
Risk of data remanence
Data remanence refers to residual representation of data that remains on storage media after erasure or attempts to delete it. Because of data remanence, erased files on flash storage may be recoverable by specialized methods.
When flash storage erases data, it marks cells as empty and available for new writes. The high voltage erase does not completely eliminate the cell’s charge. With the right forensic tools, it may be possible to recover traces of old data from erased flash storage cells.
Limited drive writes per day
The drive writes per day (DWPD) rating indicates how much data can be written to an SSD daily over its warranty period before the drive exceeds its write endurance limits. Most consumer SSDs are built for just 0.1-0.5 DWPD.
This means a 1TB consumer SSD with 0.3 DWPD can withstand around 300GB of data writes per day. Heavy workloads well beyond 0.5 DWPD require more durable and expensive enterprise SSDs designed for 1-10 DWPD.
Vulnerabilities to electrical shock
Electrical shocks, fluctuations, and surges can damage SSD controller electronics as well as affect the charge state of storage cells. Drops in voltage can cause the drive to reset and lose temporary cached writes. Power spikes can potentially fry controller components.
Because they have no electronic components, HDDs are more resistant to electrical events that SSDs. A magnetically induced voltage from an MRI machine or lightning strike is less likely to harm HDD internals compared to SSD electronics.
Fragmentation over time
Although flash storage reads and writes data very quickly overall, one downside is that performance may degrade slightly over time as fragmentation increases. With HDDs, sequential sectors and tracks allow contiguous data to be written sequentially. Flash storage accesses data more randomly.
When data is rewritten, flash storage controllers have to find available free pages spread throughout different blocks to save the data. This can lead to increased fragmentation of data over time. Higher fragmentation means more time is needed for garbage collection to free up usable space.
Not suitable for archival storage
The limited write endurance and potential for data remanence make SSDs a poor choice for long-term archival storage. Archived data that is rarely accessed will just sit on flash storage cells losing charge over time.
For infrequently accessed data that needs to be stored for years, HDDs are a better option. Magnetic charges decay more slowly than electric, so HDDs can retain data reliably for longer durations. Large capacity HDDs provide cheap and durable archival storage.
Lifespan limited by controller
The NAND flash storage cells are not the only components that can fail and limit the lifespan of a SSD. The controller electronics that manage the flash memory also have a failure risk.
Contrary to myth, SSD controllers do not last forever. The small scale electronics are subject to electromigration and thermal cycling stresses over time similar to CPU chips. Eventually, errors start occurring in the controller silicon that lead to performance issues or failure.
Not impervious to magnets
There is a common misconception that flash storage and SSDs are unaffected by magnetic fields due to having no moving parts. In fact, magnets can damage SSDs in multiple ways.
Powerful magnets can flip the electric charge in storage cells, corrupting data. Neodymium rare earth magnets can demagnetize the read/write heads used to access data. Large magnets may even physically distort the metal chassis and circuit board.
Price premium for higher capacities
The price per gigabyte for flash storage rises dramatically at higher capacities due to fabrication challenges. Producing 500GB flash storage is relatively straightforward. But scaling up to multi-terabyte capacities requires stacking dozens of flash dies, which reduces yields and increases pricing.
As a result, the cost curve is not linear. While a 500GB SSD may cost around 20 cents per gigabyte, a 4TB model will be around 50 cents per gigabyte – more than double the cost per GB despite having 8x the capacity.
Not cost effective for cold storage
If you need infrequently accessed storage for archiving cold data, flash storage is likely not the most cost-effective solution. The higher $/GB pricing along with limited write endurance makes SSDs better suited for more active hot data.
For rarely accessed cold data that just sits, HDDs provide much cheaper long-term storage. The lower media cost and virtually unlimited writes of HDDs make them better for secondary tier storage that sees only occasional reads.
Susceptible to silent data corruption
Silent data corruption refers to corrupted data that goes undetected by the storage device. This can occur if voltage fluctuations or controller errors result in incorrect data being written without errors flagged.
Silent errors are possible with both flash storage and HDDs. But the risk may be higher with flash storage due to the increased complexity of data mapping between logical addresses and physical flash pages. Undetected bit flips can lead to files being silently corrupted.
Not impervious to vibration
Although flash storage contains no moving mechanical parts, it is still susceptible to vibration damage. Solder joints between flash memory chips and circuit boards can crack under extended vibration. Connectors may also loosen over time.
Server-grade SSDs designed for enterprise environments are engineered with vibration resistance in mind. But consumer portable SSDs are generally not built to withstand extended vibration in equipment like shipping trucks or construction tools.
Mitigating the disadvantages of flash storage
While flash storage has some drawbacks, many of the disadvantages can be overcome through careful product selection or adjustments to your usage patterns:
- Choose enterprise-grade SSDs for better endurance and reliability if you have demanding workloads
- Use HDDs for rarely accessed archival data to get lower costs
- Keep a recent backup of important data to protect against unrecoverable SSD failure
- Check for firmware updates that may improve performance, fix bugs, and extend lifespan
- Avoid exposing flash storage to excess vibration, heat, or magnetic fields
- Use separate flash storage devices for operating systems vs. user data to improve lifespan
Conclusion
Flash storage provides significant speed advantages over hard disk drives, but the technology still has some limitations. The finite number of erase/write cycles, susceptibility to data loss from electrical issues, and higher costs for large capacities require consideration for certain use cases. Understanding the disadvantages of flash storage allows you to select the right storage technology for your specific needs.