According to my SSD SMART information, "Percentage lifetime used: 90%."
Does it mean the SSD's remaining lifetime is estimated to be 10% or 90% of its entire lifetime?
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It means your drive has 90% "life" left.
After some additional research, I came across this Crucial.com article that mentions the following:
In regards to the attribute named “Percentage Lifetime Used” (sometimes referred to as “Percent Lifetime Remaining”), this is simply a metric for how much wear life is left on your SSD. A solid state drive, like any flash memory-based storage device, has a limited amount of data which can be written to the memory blocks before they start to lose their reliability, and eventually go into read only mode. Your Crucial SSD will keep track of this life with SMART attribute 173, “Average Block Erase Count.” The Lifetime Used is a reflection of the block erase count in terms of a percentage. For example if your drive is rated for 3000 block erases and you have a total of 100, your Percentage Lifetime Used would be 100/3000, or 3-4%. For percent lifetime remaining we would simply take (3000-100)/3000 = 96-97%. These attributes are not a full picture of the health of a drive, but an expectation of how much usable life is left.
Because the attribute is vendor-defined, it can be complicated and confusing to figure out how each drive implements it.
Try running the Crucial Storage Executive tool to get a more accurate SMART reading that (should) follow Crucial's definitions by the book.
Turns out that the screenshot I had included in the question was a bit old. A more recent version of the screenshot shows that the "Percent Lifetime Used" is now down to 84%, which corroborates MMM's most recent answer stating that Percentage Lifetime Used” = how much wear life is left on your SSD (higher is better):
Use the diagnostic software that came with your drive..I use them and a payware program named Hard Disk Sentinel on all my WD SSD 500mb-1TB Drives in my rigs. Anyway 90% or what ever % means HOW MUCH LIFE YOU HAVE LEFT.
I don't use my SSD's for gaming or storage only for OS's and productivity software like Security,Diagnostic, Tool or business type software NO games or media etc. For example one of my drives after 4 years of use now has 97% life expectancy so you must you your drive as either a main drive or for games or something like storage or server use that constantly writes to the disk in big chunks. Writing to an SSD is what wears it out.
Use a real app or payware program that is more accurate not these lame free ones like Crystal etc that ONLY show SMART a good program shows every single parameter not just SMART.
Or use the one that came with your drive its usually the most accurate as its made for the exact drive you own. No offense but how could you ever think it was the other way round? 90% used? LOL! That would be impossible unless you write TB's of data a day or your drive is broken.
In any case SSD's are common or the norm today and their cheap to boot so when it gets close to the end just buy another one or a bunch of em I always a have a few SSD's as backups in storage just in case etc. Again the percentage is ALWAYS just the life left nothing more its why they all say GOOD. You be concerned when they say BAD. Just think of it as a fuel gauge when it hits 10-5% time to buy or use a new one. : )