Does 3D TLC BiCS require occasional rewrites to preserve the state of each memory cell? I know planar TLC was supposed to have that sort of issue and it adversely impacted long-term data storage if it was in an infrequently used device. In fact, does any 3D TLC have that problem these days?
It's temperature dependent. Put any nand in your freezer and it's good for decades or centuries. Put it in a 70C chassis and electrons leak a lot faster and may need refreshes during its service life.
It a little more complex than that. I've seen a couple breakdowns of read/write wear and data retention rates at various temperatures from -20C to 70C. Can't find the link now but, ideally you should write 'hot' and store 'cold' for maximum data retention, but temperatures too low have been shown to corrupt NAND as well soo probably more like store cool, like refrigerator temps.
Like hojnikb says voltage drift will happen to any NAND given time (depending on a number of factors). It is just that TLC NAND (especially at small lithographies) suffers more noticeably from this since it doesn’t have the margins that MLC NAND or SLC NAND do (everything else being equal).
That SSDs using 3D TLC NAND may need to do some rewrites in order to keep everyting in shipshape is something I have seen personally (though under a bit more challenging conditions than normal perhaps).
The 3D TLC NAND Intel/Micron makes would likely be one of the more easily affected since it looks like they may have gone with a 20nm lithography rather than something like 40nm like Samsung did (don’t know what lithographies Toshiba/WD or SK Hynix have gone with though).
Regarding the 3D TLC NAND from Toshiba/WD I think it shouldn’t need to rewrite files much except for in challenging conditions (or if the data has been on the drive a fairly long time). Unless of course there is either something very wrong with its construction or it is paired with an inept controller and/or using poorly written algorithms.
Thanks all for the thoughts on the matter. I've been pretty reluctant to buy TLC-based drives because of the data loss/rewrite situation because I tend to use one computer or another and then abandon it for weeks or sometimes months -- usually seasonally since I run larger hotter systems in the winter and then switch to cooler systems in the summer. It means I'll put aside one laptop or another for a good 5-6 months and the need for TLC to have occasional rewrites seemed like it might have meant blowing the dust off a computer I'd otherwise forget about for a long time. It sounds like its an unfounded worry.
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BrokenCrayons - Thursday, February 23, 2017 - link
Does 3D TLC BiCS require occasional rewrites to preserve the state of each memory cell? I know planar TLC was supposed to have that sort of issue and it adversely impacted long-term data storage if it was in an infrequently used device. In fact, does any 3D TLC have that problem these days?hojnikb - Thursday, February 23, 2017 - link
Every flash device needs this, but some need a lot longer to degrade.saratoga4 - Thursday, February 23, 2017 - link
It's temperature dependent. Put any nand in your freezer and it's good for decades or centuries. Put it in a 70C chassis and electrons leak a lot faster and may need refreshes during its service life.Ej24 - Friday, February 24, 2017 - link
It a little more complex than that. I've seen a couple breakdowns of read/write wear and data retention rates at various temperatures from -20C to 70C. Can't find the link now but, ideally you should write 'hot' and store 'cold' for maximum data retention, but temperatures too low have been shown to corrupt NAND as well soo probably more like store cool, like refrigerator temps.evancox10 - Thursday, February 23, 2017 - link
With 3D flash, they raised the bit cell size back up, so reliability should be better. Smaller cells are more susceptible to that (and other) issuesGlaring_Mistake - Thursday, February 23, 2017 - link
Like hojnikb says voltage drift will happen to any NAND given time (depending on a number of factors).It is just that TLC NAND (especially at small lithographies) suffers more noticeably from this since it doesn’t have the margins that MLC NAND or SLC NAND do (everything else being equal).
That SSDs using 3D TLC NAND may need to do some rewrites in order to keep everyting in shipshape is something I have seen personally (though under a bit more challenging conditions than normal perhaps).
The 3D TLC NAND Intel/Micron makes would likely be one of the more easily affected since it looks like they may have gone with a 20nm lithography rather than something like 40nm like Samsung did (don’t know what lithographies Toshiba/WD or SK Hynix have gone with though).
Regarding the 3D TLC NAND from Toshiba/WD I think it shouldn’t need to rewrite files much except for in challenging conditions (or if the data has been on the drive a fairly long time).
Unless of course there is either something very wrong with its construction or it is paired with an inept controller and/or using poorly written algorithms.
BrokenCrayons - Friday, February 24, 2017 - link
Thanks all for the thoughts on the matter. I've been pretty reluctant to buy TLC-based drives because of the data loss/rewrite situation because I tend to use one computer or another and then abandon it for weeks or sometimes months -- usually seasonally since I run larger hotter systems in the winter and then switch to cooler systems in the summer. It means I'll put aside one laptop or another for a good 5-6 months and the need for TLC to have occasional rewrites seemed like it might have meant blowing the dust off a computer I'd otherwise forget about for a long time. It sounds like its an unfounded worry.Samus - Friday, February 24, 2017 - link
Don't leave your laptop in the sun for weeks at a time without powering it on.