Unused blocks vsd TRIM [was Re: Working on Discard for FFS]

> The semantics of fdiscard(2) are a bit messy, with TRIM and undefined
> contents.

The messy bits apply only when discarding space on a device, as I read
the (9.1) manpage.  I suspect that's because that's how devices work:
dropped blocks return, essentially, whatever the implementation finds
most convenient.  Requiring anything else would mean it simply couldn't
work on existing devices.

> That's surprising to me, as I see telling the hardware that blocks
> are no longer needed seems separable from a file no longer
> referencing those blocks.

Conceptually, so do I.  But, on a device with a filesystem on it, when
discarding part of a file, the blocks are no longer used, so they might
as well be discarded; when discarding blocks, they'd better no longer
be part of a file.  So tying them together makes at least some sense
when discarding space in a filesystem file.

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