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Comment by jnwatson | Hacker News Reader

Perhaps I'm misunderstanding you, but trim doesn't secure delete anything. It merely indicates to the SSD that the sector is unused so that it can reap the underlying NAND location the next time it decides to garbage collect.

In other words, the data is still there in the flash, but only the SSD firmware (and physical access) has access to it.

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I think the reality is that some blocks will be lazily erased. I believe modern flash firmware periodically GCs in the background to prevent write amplification. If this is triggered you may overwrite data.

NAND requires an erase before write so I wouldn't be surprised if some controllers are lazily erasing blocks to get better long term write speeds and prevent GC hiccups.

At least Intel (and probably others) now provide full-disk encryption by default. So the flash is unreadable by any other controller: http://www.intel.com/content/dam/www/public/us/en/documents/...

The reason for marking it as erased is so the firmware can physically erase the flash sector. It could happen immediately, a minute later or next week. But a well-written SSD firmware will try to erase blocks any time it's not busy doing something else, as erasing blocks takes way longer than writing them.

You might be able to recover something from the physical flash, but there's definitely no guarantees.

> erasing blocks takes way longer than writing them

Honest question, could you elaborate on that? Intuitively I would've thought writing and erasing are _the same_ from a physical standpoint, insofar as "erasing" means writing zeros.

There are two operations on flash memory: "erase" (set all bits to 0) and "write" (set some bits to 1). The former is expensive, the latter cheap. Note that "write" can't set bits to 0. Hence the benefit of keeping zeroed blocks around.

That makes perfect sense, thank you.

For flash, erase resets an erase unit to its default state, which can be all 0 or all 1 depending on the technology. Writes changes bits from the default to the opposite only. Depending on the flash chips interface, you may be able to do this at the bit, byte, or block level, but changing in the opposite direction is expensive and time consuming.

In theory we could make flash with tiny erase units (down to the bit level), but in practice we don't because the extra circuitry would drive the price through the roof.

That's interesting, if nanotech (the part that's about assembling 'things' at a molecular level, hyper-grained 3D printing so to speak) really enters the economic breakthrough we've been expecting since the early 80's, I see one huge improvement for flash right here.

I found this article about recovering data from SSDs very interesting.

https://news.ycombinator.com/item?id=11608568

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