4 ms·

What happens if the remaining space is TRIMMED but routinely accessed? (for example by dd, read only)

• wtallis 2y ago
  If a logical block address is not mapped to any physical flash memory addresses, then the SSD can return zeros for a read request immediately, without touching the flash.
  • stavros 2y ago
    Does the mapping happen on first write? Is TRIM then a command that signals the SSD to unmap that block?
    • BeeOnRope 2y ago
      Yes.
• Dwedit 2y ago
  You read zeroes.
  • BeeOnRope 2y ago
    Not guaranteed by default for NVMe drives. There's an NVMe feature bit for "Read Zero After TRIM" which if set for a drive guarantees this behavior but many drives of interest (2024) do not set this.
    • Dwedit 2y ago
      Hmmm, when I quick-formatted a drive (which TRIMs the whole thing), then tried reading it back in a disk hex editor, I just saw zeroes.
      • BeeOnRope 2y ago
        It seems like that would be a likely common behavior for the FTL, but other options are possible (e.g., reading the old blocks) and it wasn't guaranteed by the spec, which is why they added this NVMe flag (so-called "DZAT") so that you can actually rely on it.