Solid state drives (SSDs) have become increasingly popular in personal computers and data centers due to their fast speeds and lack of moving parts. However, SSDs have a limited number of write cycles before they can no longer store data reliably. This article will examine what happens when an SSD reaches its write limit.
How do SSDs work?
SSDs store data on NAND flash memory chips rather than magnetic platters like traditional hard disk drives (HDDs). The NAND flash memory contains cells that trap electrons to represent data as either a 1 or 0. When new data needs to be written, the cells must be erased before electrons can be trapped in a new configuration. This process of erasing cells before rewriting is called program/erase cycling.
Each cell in an SSD has a limited number of program/erase cycles it can endure before it can no longer reliably store data. Most consumer SSDs are rated for anywhere from 500 to over 5,000 program/erase cycles. Once a cell reaches its limit, it becomes unreliable and may result in data errors.
What is the write limit of an SSD?
The write limit of an SSD refers to the total amount of data that can be written to the drive before its cells wear out. This is determined by two factors:
- Program/erase cycle limit – As mentioned above, each NAND flash cell has a finite number of times it can be programmed and erased. This determines the total number of writes possible.
- Drive capacity – Larger capacity SSDs naturally have a higher total write limit because they have more NAND flash cells.
For example, a 256GB SSD with 3000 program/erase cycles would have a higher total write limit than a 128GB SSD with 3000 cycles. The 256GB drive has twice the number of cells.
Write limits are commonly measured in terabytes written (TBW). Entry-level consumer SSDs may have as little as 100 TBW, while high-end data center models can exceed 10,000 TBW.
How is the write limit tracked?
The SSD controller tracks how many program/erase cycles each cell has endured. This keeps a running tally of the total writes across the drive over its lifetime. There are two metrics tracked:
- Total Terabytes Written (TBW) – Tracks the cumulative number of terabytes of host writes to the SSD.
- Wear Leveling Count – Tracks the number of program/erase cycles on each NAND flash cell.
Advanced SSDs use wear leveling algorithms that distribute writes across all available cells evenly. This prevents any one cell from wearing out prematurely. The wear leveling count tracks this process.
Some SSDs also have spare blocks that can be substituted for worn out blocks to extend the usable lifetime.
What happens when the write limit is reached?
There is no single point where an SSD just stops working once the write limit is reached.Performance and reliability will slowly degrade. There are a few things that can occur as the drive approaches its end of life:
- Increased read latencies – As more cells wear out, read operations take longer as error correction has to be performed.
- Increased write latencies – Program and erase operations take longer as more cells reach their cycle limit.
- Bad blocks – Unusable blocks that are retired by the SSD controller will reduce available capacity.
- Data errors – As more cells wear out, uncorrectable errors will occur during reads resulting in data loss.
Most SSD controllers will continue retrying read and write operations, so the drive will appear to be slow rather than fail outright. However, storage capacity and performance will gradually decline.
Read disturb errors
In addition to program/erase cycling, SSD cells are also subject to read disturbs. When a read occurs, nearby cells in the same block can experience voltage fluctuations that over time reduce their reliability. SSD controllers thus track read operations and refresh or retire blocks that have exceeded read disturb limits.
As an SSD nears its write limit, read disturbs are more likely to exceed error correction capabilities resulting in uncorrectable errors during reads. This contributes to the overall decline in reliability and performance.
How is end of life predicted?
To provide warning of an SSD reaching its write limit, most drives report two SMART attributes:
- Percentage Used – Derived from the Total Terabytes Written and drive capacity. Gives a rough percentage indicating write lifespan consumed.
- Wear Leveling Count – The minimum and maximum program/erase cycles for any cell in the drive.
Monitoring these values allows estimating the remaining lifespan. The percentage used steadily increasing towards 100% indicates the write limit is being approached. The wear leveling minimum and maximum values converging means the cells are nearing their cycle limits.
Specific thresholds depend on the make and model. But once percentage used exceeds 90-95% warnings should be heeded. And if the wear leveling minimum and maximum differ by less than 10%, the SSD has nearly exhausted its write endurance.
Maximizing SSD lifespan
To maximize the usable lifetime of an SSD:
- Enable the TRIM command if supported – Allows the SSD to efficiently erase deleted blocks
- Minimize writes – Store temporary files on RAM rather than the SSD
- Avoid full disk encryption – The constant writes of encrypted data reduces SSD lifespan
- Use drive sparingly – The fewer writes to the SSD, the longer it will last
- Monitor SMART attributes – Watch for percentage used and wear leveling values approaching limits
Recovering data from a worn out SSD
Once an SSD has exceeded its write limit, data recovery becomes challenging. The drive will have high error rates making normal reads unreliable. However, a few methods may work to recover some data:
- Use disk cloning software – This makes a full byte-for-byte copy of the drive ignoring errors. Useful data can then be extracted from the clone.
- Low level disk editors – Allow manually reading and parsing the SSD firmware to find readable blocks and extract data.
- Data recovery services – Expensive, but may physically repair the NAND chips to temporarily enable data extraction.
If the SSD is encrypted, data recovery is generally impossible once the drive begins failing beyond its endurance limits.
Replacing a worn out SSD
Once an SSD has exceeded its write limit, it should be replaced. Continued use risks the drive completely failing. Check the warranty coverage first – consumer SSDs often have 3-5 year warranties covering write endurance. If covered, the manufacturer will replace the worn out drive.
If outside the warranty coverage, purchase a new SSD. Be sure to get a model rated for higher endurance – consumer models with 100 TBW ratings generally have short 2-3 year lifespans for heavily used drives. For longer lifespans, look for SSDs with 1,500+ TBW ratings.
Before replacing the SSD, be sure to back up any data. With the drive already past its reliability limits, a failure could occur at any time.
Conclusion
SSDs have a finite write limit determined by the program/erase cycle endurance of their NAND flash memory cells. This causes several issues as the drive exceeds its rated endurance – increased latencies, bad blocks, and data errors. Performance and capacity slowly degrade until the SSD completely fails.
Monitoring the Total Terabytes Written and Wear Leveling Count SMART attributes can provide warning to replace the drive before failure. With proper care and maintenance, SSDs can provide years of reliable high speed storage. But exceeding write limits will inevitably shorten their usable lifespans.
