In the past week, quite a few media outlets have posted articles claiming that SSDs will lose data in a matter of days if left unpowered. While there is some (read: very, very little) truth to that, it has created a lot of chatter and confusion in forums and even I have received a few questions about the validity of the claims, so rather than responding to individual emails/tweets from people who want to know more, I thought I would explain the matter in depth to everyone at once. 

First of all, the presentation everyone is talking about can be found here. Unlike some sites reported, it's not a presentation from Seagate -- it's an official JEDEC presentation from Alvin Cox, the Chairman of JC-64.8 subcommittee (i.e. SSD committee) at the time, meaning that it's supposed to act as an objective source of information for all SSD vendors. It is, however, correct that Mr. Cox works as a Senior Staff Engineer at Seagate, but that is irrelevant because the whole purpose of JEDEC is to bring manufacturers together to develop open standards. The committee members and chairmen are all working for some company and currently the JC-64.8 subcommittee is lead by Frank Chu from HGST.

Before we go into the actual data retention topic, let's outline the situation by focusing on the conditions that must be met when the manufacturer is determining the endurance rating for an SSD. First off, the drive must maintain its capacity, meaning that it cannot retire so many blocks that the user capacity would decrease. Secondly, the drive must meet the required UBER (number of data errors per number of bits read) spec as well as be within the functional failure requirement. Finally, the drive must retain data without power for a set amount of time to meet the JEDEC spec. Note that all these must be conditions must be met when the maximum number of data has been written i.e. if a drive is rated at 100TB, it must meet these specs after 100TB of writes.

The table above summarizes the requirements for both client and enterprise SSDs. As we can see, the data retention requirement for a client SSD is one-year at 30°C, which is above typical room temperature. The retention does depend on the temperature, so let's take a closer look of how the retention scales with temperature.

EDIT: Note that the data in the table above is based on material sent by Intel, not Seagate.

At 40°C active and 30°C power off temperature, a client SSD is set to retain data for 52 weeks i.e. one year. As the table shows, the data retention is proportional to active temperature and inversely proportional to power off temperature, meaning that a higher power off temperature will result in decreased retention. In a worst case scenario where the active temperature is only 25-30°C and power off is 55°C, the data retention can be as short as one week, which is what many sites have touted with their "data loss in matter of days" claims. Yes, it can technically happen, but not in typical client environment.

In reality power off temperature of 55°C is not realistic at all for a client user because the drive will most likely be stored somewhere in the house (closet, basement, garage etc.) in room temperature, which tends to be below 30°C. Active temperature, on the other hand, is usually at least 40°C because the drive and other components in the system generate heat that puts the temperature over room temperature.

As always, there is a technical explanation to the data retention scaling. The conductivity of a semiconductor scales with temperature, which is bad news for NAND because when it's unpowered the electrons are not supposed to move as that would change the charge of the cell. In other words, as the temperature increases, the electrons escape the floating gate faster that ultimately changes the voltage state of the cell and renders data unreadable (i.e. the drive no longer retains data). 

For active use the temperature has the opposite effect. Because higher temperature makes the silicon more conductive, the flow of current is higher during program/erase operation and causes less stress on the tunnel oxide, improving the endurance of the cell because endurance is practically limited by tunnel oxide's ability to hold the electrons inside the floating gate.

All in all, there is absolutely zero reason to worry about SSD data retention in typical client environment. Remember that the figures presented here are for a drive that has already passed its endurance rating, so for new drives the data retention is considerably higher, typically over ten years for MLC NAND based SSDs. If you buy a drive today and stash it away, the drive itself will become totally obsolete quicker than it will lose its data. Besides, given the cost of SSDs, it's not cost efficient to use them for cold storage anyway, so if you're looking to archive data I would recommend going with hard drives for cost reasons alone.

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  • npz - Thursday, May 14, 2015 - link

    I guarantee you it is NOT common knowledge at all. Very very few people now are aware of limited data rentention issues inherent of SSDs. In fact people assume that greater endurance rating == better when in fact higher endurance DECREASES retention ability.

    If it was common knowledge why do you think people have been saying mechanical hard drives are becoming obsolete and soon we'll all be going to SSDs?
    Reply
  • TheRealPD - Thursday, May 14, 2015 - link

    Working in reverse order, I don't recall anyone reputable saying that HDDs were going to become obsolete within the timescale that NAND's likely to be used in SSDs for - so sources please.

    That's not to say, of course, that there aren't uses where NAND based SSDs will dominate for a while, however this hasn't been talked about in the sense of replacing offline backup media as the costs are prohibitive for the consumer, so are we really suggesting that people would simply not use a PC, laptop, netbook, tablet, whatever with critical data on for a year or more on the consumer end?

    As to it being common knowledge, what I said was that it was common knowledge back in 2010/11 - & if people since then haven't looked at the pros & cons of the tech then it 'could' be argued more fool them. Well, searching for info is what people did back in the day which is why i knew about it.

    However, the functional issue here is websites (not including Anand since, as Kristian Vättö noted after i'd posted, the charts have been used on here before now) making out that there's suddenly been this new revelation from JEDEC/Seagate - when it's well over 4.5 years old & the info's been online for all of the time.

    Now, what you 'should' expect from a decent PC website is the provision of accurate information, & that includes them both being aware & making their readership aware of what the standards are where they effect potential usages (albeit that the cost per GB still makes HDDs far more economical for backup/offline storage), so if we accepted that individuals might not search for information about the agreed min specs, the sites that have both been publishing this as 'news' & have also been reviewing SSDs (so claiming expertise) are fundamentally to blame for not maintaining this information in the consumers' minds...

    ...but, rather than sticking their hands up & saying "mea culpa, the info's been there for years but we didn't tell you", they're instead pretending that it's a new issue that's only just been discovered.

    Now, if someone didn't know, well, now they do - & it's hardly a situation that makes NAND based SSDs unusable or that there's no alternative (& much cheaper) storage media available - but if you didn't then perhaps it's both worth considering where you get your tech information from in the future & remembering that an onus on the individual to understand the limitations of any tech, esp when the info's readily available online.
    Reply
  • Cerb - Saturday, May 16, 2015 - link

    "If it was common knowledge why do you think people have been saying mechanical hard drives are becoming obsolete and soon we'll all be going to SSDs?"

    Because we hope the prices to come down, and capacities to go up. I don't see why any of it being common knowledge would or should change that. Mainstream HDDs aren't exactly built for cold storage, you know, and commonly fail not long after being powered back up fr awhile, or even if remaining active, shortly after moving an old PC.

    If you wrote 10k over every cell on a eMLC SSD, in just a few years, you managed to write an amount of data that likely would have been *IMPOSSIBLE* with spinners, in a practical application. So what if it only lasts 3 months once you power it off? You're already trying to get the last bits of unknowns taken care of for their replacements, if you haven't already started rolling them out. Likewise, if you wrote 3k to your regular MLC SSD, you've been beating the crap out of it, and have will have been so much more productive than would have been possible with an HDD that you really shouldn't care that it can't be re-purposed as an archival device.
    Reply
  • triarius - Wednesday, May 13, 2015 - link

    So the one week figure comes from when what is effectively the room temperature is 55C and the running temperature is 25C. Someone must have a pretty powerful case cooling system to achieve that. Reply
  • mkozakewich - Wednesday, May 13, 2015 - link

    I know, I lol'd at the chart. Those areas are greyed out for a reason, but it's interesting they included those instead of stripping them out. Reply
  • zmeul - Wednesday, May 13, 2015 - link

    quote: "In a worst case scenario where the active temperature is only 25-30°C"

    you obviously don't do summers

    ---

    let's say you have a you have a SSD with data and you need to ship it internationally, across the ocean(s) via plane/boat
    will the data be readable and intact once it reaches it's destination? knowing what we know and depending on how much the item will get stuck in customs improper storage, data integrity will be affected
    Reply
  • Murloc - Wednesday, May 13, 2015 - link

    that's a stupid way to send data.

    Still, airplane holds are cold and planes are fast so there is no issue.
    Reply
  • zmeul - Wednesday, May 13, 2015 - link

    except when your item get stuck in customs for 1 month?! Reply
  • mkozakewich - Wednesday, May 13, 2015 - link

    That's only four weeks. If you're shipping an SSD, you probably shouldn't have written 100 TB of data to it. Reply
  • melgross - Wednesday, May 13, 2015 - link

    Tell that to Microsoft. In order to upload all of your data to Asure, if you can't do it online because of the amount of data and connection reliability, they tell you to send your drives to them, and they will do it, returning your drives afterwards.

    Hopefully these drives will be sent by a very fast carrier, but many people in IT aren't the brightest. Saving money can be more important to them. Knowing this might help them make the right decision.
    Reply

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