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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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