Recently, some tech sites on the web reported on a phenomenon affecting SSDs which could purportedly result in loss of data, particularly on devices which are stored in an unpowered state for periods of time.
In our industry, the ability to keep data for specified periods of time is referred to as data retention. It’s an attribute which is measured, evaluated, and engineered extensively by our design teams. Our products must undergo very rigorous data retention testing before we’ll release them to volume production. Both our NAND Flash and SSD organizations are fully engaged on this topic. It comes up frequently in our discussions with our customers, it’s addressed in our component data sheets, and it is well-understood by data storage system designers and even some end users.
So, let’s talk about data retention a bit. But first, let me give you the bottom line: You shouldn’t worry about data retention! As I like to say, Micron has your back. Our SSDs are designed with great rigor, and we work with our customers to help them fully understand the physical characteristics of our products and the specific needs of their system environment.
JEDEC and the Industry Standards for Data Retention
We have to start this discussion with a mention of our colleagues at JEDEC. JEDEC is the Joint Electron Device Engineering Council. This is a trade association and engineering standards board for the semiconductor products industry. Micron is a contributing JEDEC member, and many of our team members hold leadership roles within the organization. When NAND flash devices were first being developed, JEDEC was studying data retention and developing specifications which gave manufacturers and their customers added assurance about the performance of NAND flash-based products.
The notion that a NAND-based storage device will retain data for a bounded time period under very specific environmental conditions is not a new one and, in fact, it is very well-defined. What is also well-understood is that the data retention attribute changes over the lifetime of a NAND flash-based device. When a NAND flash device is new from the factory, it can retain data in an unpowered state (at specific temperature conditions) for many years. However, data retention specs are not given for a new device. It is very important to note that published specifications for data retention describe the behavior of the device only at the end of its prescribed service life.
The JEDEC specification for data retention tells us that for enterprise storage devices, data retention at the end of the service life shall be at least three months (stored at 40°C). For SSDs in the client computing market, data retention shall be at least one year after the drive’s service life (assuming it’s stored at 30°C). At the SSD level, this service life is specified in total bytes written, or TBW. For client SSDs, TBW ratings range from tens to hundreds of terabytes (1012 bytes), whereas for enterprise drives, TBW ratings are in the petabyte range (1015 bytes) and higher.
The Close Relationship of Endurance and Data Retention
TBW ratings are also referred to as endurance ratings. Endurance and data retention are very strongly linked. If the SSD in your server or data center has a lifetime endurance rating of 7 petabytes written, this does not mean that the SSD will fail when it writes that 8th petabyte. Rather, it means that when that 7th petabyte is written, the data retention in an unpowered state at a specific temperature (40°C or 104°F) is down to that three-month enterprise specification.
Similarly, for a client SSD, JEDEC set the data retention spec at one year, which allows the user much more time to go back to get data from an unused device, if needed. Again, keep in mind that this one-year spec only applies to the end of the SSD’s lifetime. When the SSD is new or of modest age, measured in TBW, the data retention is much longer. In our experience, it is very rare for the typical Windows or MacOS user to get anywhere near the TBW specification during the lifetime of the host computer system. In almost all client computing cases, the end user can be comfortable knowing that the SSD should have several years of data retention, until the drive approaches the end of the drive service life.
Data storage system designers well understand this phenomenon. The implication is that old and well-worn NAND flash storage devices are not intended for archival data storage. At the end of a device’s service life, the device should be retired and the data onboard should be transferred to a new device. The three-month JEDEC spec is set specifically to give the system administrator plenty of time to accomplish this data transfer, if necessary.
System designers have long known that NAND-based devices aren’t optimal for archive storage. The primary reason for this is that NAND flash is quite expensive compared to magnetic media, especially tape, which is used as archive media even today. But the secondary reason is that after extensive use, the unpowered data retention decreases as I’ve described here.
What Does This Mean to You?
All of this is not intended to say that there are no concerns with regard to data retention. There are some factors that can make data retention worse, but this also means that there are mitigating actions you can take to optimize data retention:
- It’s true that storing NAND flash devices at very hot temperatures will reduce data retention, especially for those older, well-used devices. So, take the common sense approach and store your devices at reasonable temperatures, ranging from room temperature up to 40°C/104°F for enterprise drives or 30°C/86°F for client SSDs.
- Really, it’s not a good idea to store any electrical or electro-mechanical device at very high temperatures for extended periods of time. In this sense, SSDs are not unique. Your car in the summer time is never a good storage location. However, if you leave your laptop in your car on a hot afternoon occasionally, you don’t have to panic about data retention, particularly if your device is new or near new.
- It’s an interesting fact that data which is written on NAND flash media in a hot environment has longer data retention than data written at moderate or cold temperatures. So, the fact that the operating temperature in your data center may be hot is not a concern at all—at least as long as the device temperature doesn’t exceed 70°C/158°F.
- Be aware that when your device is brand new, its data retention is much longer than the JEDEC spec; but do monitor the age of your drive, so you can be aware of the associated reduction in data retention. A good way to monitor the life of your Micron or Crucial SSD is to use our Storage Executive software to examine the SSD’s SMART attributes. In particular, our Attribute 202 reflects the Percentage of Lifetime Used for the SSD. When this attribute hits 100%, you know that you’ve hit that three-month data retention point for enterprise SSDs or the one-year point for client SSDs.
- To be sure, there is no electronic or electro-mechanical device that has infinite data retention. Some storage media have longer data retention than others, and some media types are more expensive than others. There is always a need to balance cost, performance, and retention properties when picking an archival storage device. There are some circumstances where NAND flash could be an acceptable archive media (particularly if speed is an issue), but in many cases, magnetic disk and tape could provide a better solution. NAND flash is a better solution in a powered-on, “cold storage” application.
- Finally, we are very proud of the quality and reliability of our NAND and SSD products, but even the best devices fail sometimes. Even if we never had to talk about data retention, you should always keep backups of your important data and regularly check the integrity of your backups.
If you still have questions regarding data retention, we have technical sales and support staff available to answer your questions. In addition, we’d love to hear your feedback. You can connect directly with me on Twitter: @JTanguyFlash or via our @MicronStorage handle.