title="Discard for FFS"
The fdiscard system call, and the file-system-level VOP_DISCARD operation
backing it, allow dropping ranges of blocks from files to create
sparse files with holes.
This is a simple generalization of truncate.
Discard is not currently implemented for FFS; implement it.
(For regular files.)
It should be possible to do so by generalizing the existing truncate
code; this should be not super-difficult, but somewhat messy.
Note that it's important (for both the WAPBL and non-WAPBL cases) to
preserve the on-disk invariants needed for successful crash recovery.
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