TRIM / Discard

Deleting a file only updates the filesystem journal. The SSD still believes those LBAs are valid until the OS sends a discard. TRIM is that courtesy call: these logical addresses are free; you may erase the underlying pages when convenient.

On SATA you will see DATA SET MANAGEMENT flow through the AHCI driver; on NVMe it is Dataset Management opcode 09h with AD=1. Same intent, different packet shapes—both documented in the NVMe and ATA worlds linked from the SSD and SATA cards.

Enable periodic discard on client SSDs; understand that TRIM assists garbage collection, not compliance wiping. For crypto erase or sanitize, use the explicit secure-erase commands vendors document separately.

Revision 1.0c, dated October 3rd, 2022, shows a maximum case of 256 ranges in Figure 41, Range 0 through Range 255, with Range 255's Starting LBA at bytes 4095:4088. Number of Ranges (NR), Command Dword 10 bits 07:00, is a 0's-based count of 16 byte range sets, and the length in logical blocks field is 1-based. If the controller reports a non-zero Dataset Management Ranges Limit (DMRL), it does not process attributes or context attributes for any range number greater than or equal to that value. A non-zero Dataset Management Range Size Limit (DMRSL) means the controller does not process attributes or context attributes for a logical block whose LBA offset from the starting LBA of the range is greater than or equal to that value. A non-zero Dataset Management Size Limit (DMSL) is a processing limit on the total number of logical blocks for the command. DMRL, DMRSL, and DMSL are all non-zero or all 0h. An unused DMRL set to all bits 1 still leaves 255 of the 256 ranges, because DMRL is 1-based and NR is 0's-based. Section 3.2.3.1.1 prints DMRL 1 or 2, DMSL 1,048, range 0 of 1,024 logical blocks, range 1 of 512 logical blocks, and the first 24 logical blocks of range 1 only as an example, and only when ONCS bit 2 is 1 and DMRSL is at its maximum. When the Deallocated or Unwritten Logical Block error is not enabled, a read is all bytes 0h if DLFEAT bits 2:0 are 001b, all bytes FFh if they are 010b, or either all 0h or all FFh if they are 000b, and that value stays the same until a write. [trim-nvme-nvm-cmdset]