I did some light reading. I see claims that wear leveling only ever writes only to zeroed sectors. Let me get this straight:
If I have a 1TB ssd, and I write 1TB of SecretData, and then I delete and write 1TB of garbage to the disk, it's not actually holding 2TB of data, with the SecretData hidden underneath wear leveling? That's the claim? And if I overwrite that with another 1TB of garbage it's holding, what now, 3TB of data? Each data sequence hidden somehow by the magic of wear leveling?
Skeptical Ruaraidh is skeptical. Wear leveling ensures data on an SSD is written to free sectors with the lowest write count. It can't possibly be retaining data if data the maximum size of the device is written to it.
I see a popular comment on SO saying you can't trust dd on SSDs, and I challenge that: in this case, wiping an entire disk by dumping /dev/random must clean the SSD of all other data. Otherwise, someone's invented the storage version of a perpetual motion device. To be safe, sync and read it, and maybe dumb again, but I really can't see how an SSD world hold more data than it can.
dd if=/dev/random of=/dev/sdX bs=2048 count=524288
If you're clever enough to be using zsh as your shell:
repeat 3 (dd if=/dev/random of=/dev/sdX bs=2048 count=524288 ; sync ; dd if=/dev/sdX ba=2048)
You reduce every single cell's write lifespan by 2 times; with modern life spans of 3,000-100,000 writes per cell, it's not significant.
Someone mentioned blkdiscard. If you really aren't concerned about forensic analysis, this is probably the fastest and least impactful answer: it won't affect cell lives by even a measly 2 writes. But it also doesn't actually remove the data, it just tells the SSD that those cells are free and empty. Probably really hard to reconstruct data from that, but also probably not impossible. dd is a shredding option: safer, slower, and with a tiny impact on drive lifespan.