Long term data retention - How is it handled?

[Hulk]

So we know that SLC SSD's have better endurance and data retention than MLC drives, which are in turn better than TLC.

But how about files that are written to the drive once and then not rewritten for years? I'm specifically talking about operating system files and applications not to mention media files.

Will the data retention last 3 year? 5 years? 10 years? And what happens when those blocks begin to not to be able to be read? Long wait times? Crashes for OS files? Flaky system behavior?

Finally, does the firmware in drives keep track of when a certain block was written and know to rewrite it after a certain period of time? Perhaps every 6 months or something? It seems as though this type of behavior would keep data fresh, as long as the drive is powered, without really affecting endurance in a significant manner.

Also, couldn't this be a "fix" for the Samsung 840EVO's? Simply program the drive to rewrite data every month.

 

[CiPHER]

JEDEC JESD22-A117B specifies that at least 12 months of retention should be guaranteed, when all write cycles have been exhausted.

However, new SSDs get a minimum of 10 years of retention. Only when you begin to wear out the flash, will this drop. It will start dropping at about 10% of rated lifecycle (MWI) and drop from 10 years to 1 year during the 10%->100% used write cycles.

SLC, MLC, TLC will have the same retention when the flash is hardly worn! It will only start making a difference when the flash approaches its MWI lifespan. This is good news for Samsung basically.

The firmware does refreshes of stale data. But it can only do this by static prediction. The retention spec basically says that 99%+ of cells must adhere to the specified retention time. But individual cells may have far less retention time and thus turn as 'bad page'. Modern SSDs have RAID5 bitcorrection so even an unreadable cell will not provoke an unreadable sector. So the host will not notice anything.

Only eMLC has lower retention of just three months. But you will not find this NAND inside consumer products.

 

[Hulk]

Wow! Thanks for the great information.

By static prediction do you mean that when a cell or page reaches a certain level of difficulty in reading it will be rewritten? I mean the history of the reading difficulty of the block is not taken into account?

Sorry for the imprecise terminology I don't know the correct words.