What happens when an SSD wears out?

Since SSDs are still fairly new, there isn't a history of posts saying "I reached my writes limit on 10% of my SSD... – and here is what happened (I.e. I lost 3 directories of work files. The entire drive just died...). Maybe it starts churning as the available sectors are reduced way down - like a PC with low memory.

They tell us that these devices will only last a few years. That is frightening - THEN WHAT HAPPENS? No one seems to know. Is it "instant paperweight" or a few files here and there vanish? Or do the SSD utilities constantly monitor and warn you long before the drive dies?

It's a bit of a myth that SSDs will wear out, especially for typical desktop use. (At least, this applies to SSDs from the past few years that have proper wear-levelling.) Even if you write 7GB/day to a decent 256GB 25nm-process SSD, the flash memory should last for tens, if not hundreds, of years. It's far more likely that the controller hardware or software will fail, given the number of failures that have been reported by manufacturers and users.

In theory, it is possible to read data even after all program/erase (p/e) cycles have been used up. In fact, the JEDEC specifies that data on consumer-grade SSDs should be readable for one year after all p/e cycles have been exhausted. So the likelihood of losing data due to the drive reaching the end of its lifetime is small; it's more likely that you'll have replaced or upgraded your system by then.

What happens when an SSD fails? The same thing that happens when a mechanical hard drive fails: You recover from your backups onto a new drive, and continue on.

This article published July 2011 talks about SSD failure modes: http://www.zdnet.com/blog/bott/ssds-are-fast-but-do-they-last/3621 . He has found that they are roughly as reliable as 1TB drives. Talking to sites that use many SSDs, they found that SSDs tend to fail catastrophically, compared to HD's that degrade gracefully over time.

Like (hard) disk drives they are made from multiple sub-systems and components, and can fail in multiple ways. Some instant, some partial/incremental.

SSD drives use the S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology) systems that disk drives use to keep track of problems, and (if queried by the BIOS/OS) alert you of potential failures.

I can only share my experience specific to my Samsung 840 EVO, it lasted 4 years but I admit that I used it intensively, I ran Debian on it and it was almost full the entire time.

It died without any warning, no SMART alert nothing, one evening it displayed 0bytes of space left, so I started deleting files but it stayed at 0bytes left, I noticed that when I deleted something the SSD capacity shrank.

So I shutdown the computer and was still able one day later to recover all data.