effective use of blkdiscard(8)?

[nia <nia%NetBSD.org@localhost>]

blkdiscard(8) seems like a command in -current that's useful for regular
maintenance of SSDs.

I would assume that a regular run of:

blkdiscard -v /dev/rwd0d

would be useful to TRIM an entire SSD, obviously destructively, so would
be useful when reinstalling NetBSD.

However, what about less obvious cases?

A large file could be created, for example, with dd:

# dd if=/dev/zero of=./testfile bs=4m count=10000

Then discarded:

# blkdiscard -v ./testfile

Would this effectively mark 40GB of this drive unused to its controller?

How good are we at propgating TRIM commands through various block device
layers?

Is fdiscard() effective on a file on FFSv2 on a cgd(4) on a dk(4) wedge?

What about ZFS on a dk(4) on a cgd(4) on a dk(4) wedge?