Ashift=12 and 4Kn

[NugentS]

I have been doing a lot of reading in an attempt to understand 4Kn, 512e and ashift=12
I have a bunch of disks (Seagate Exos 12TB) that are 512/4096 logical / physical. They are in an existing pool with ashift=12
smartctl -i /dev/ada4 shows up as
Sector Sizes: 512 bytes logical, 4096 bytes physical
Which I think means formatted 512

However I understand that ashift=12 means that ZFS is using the physical sector size of 4K - but surely the data is being stored in the old 512 way

Also, anyone know what the implications of having a ashift=9 vdev (SLOG) attached to this pool would be? The Optane 900p at first glance to be seems to be 512 only with no support for 4Kn

 

[Etorix]

Sector Sizes: 512 bytes logical, 4096 bytes physical
Which I think means formatted 512

No, it means the opposite: Data is stored in 4k sectors, but the drive is willing to pretend to the OS it stores by 512 bytes (with write amplification). Ashift=12 is just what the doctor orders—and this is a pool-wide setting.
Ashift=12 for an actual 512-byte device just means reading and writing in batches of 8 sectors. Optane is byte-addressable and does not really have a "sector size" in the sense of other devices; it will work just fine.