On Wed, 23 Sep 2026, Itai Handler wrote:
> A torn sector is possible today
> -------------------------------
>
> Consumer NVMe are generally shipped with 512-byte logical blocks and
> stay that way in use. Installing Linux does not change it: partitioning
> and mkfs work inside the logical blocks the drive already exposes.
> Changing the block size is a separate low-level operation - nvme format
> with a different LBA format - which is destructive, which no installer
> performs, and which a drive may not offer at all.
>
> The machine I am writing from is one: a Samsung PM981a carrying a
> complete Ubuntu install - GPT, EFI partition, /boot, and the rest of the
> drive as LUKS over dm-crypt with LVM and ext4 on top - and it still
> reports logical_block_size = physical_block_size = 512, with no
> atomic_write_* attributes at all. The dm-crypt device on it reports a
> 512-byte logical block too.
I did some benchmarking some times ago - and all the SSDs that I tested
had worse IOPS for sub-4k writes than for 4k-aligned writes. This means
that the SSDs have 4k sectors internally and do read-modify-write cycle
for sub-4k writes.
The SATA standard doesn't specify well what 'physical block size' should
mean on SSD - the result is that some SATA SSDs report physical block size
512 and some 4k.
The SSDs have remapping table that maps logical blocks to locations in the
flash chips - and RAM is expensive, so I doubt that any SSD vendor would
put 8 times more RAM on the SSD in order to map 512-byte sectors
individually.
Mikulas
