quoted

I think one of the important design decisions that needs to be made
early on is whether it is possible to directly access some storage
that can be updated with small random writes (either a separate flash
LUN on the device, or a section of the disk that is formatted for 4kB
sectors without SMR write requirements).

This would be nice, but I looking more generally to what I call
'single disk' systems.  Several more complicated FSs use a separate
flash drive for this purpose, but ext4 expects 1 vdev, and thus only
one type of media (agnostic).  We have hybrid HDD that have flash on
them, but the lba space isn't separate, so the FS or the DM couldn't
very easily treat them as 2 devices.

Adrian,
What if vdev that has been exposed to ext4 composed out of md device
instead of regular block device ? In other words, how do you see that
these changes in EXT4 file system apply on software RAID array of SMR
drives ?

--Alireza