> growfs: there's a growfs tool in the Netbsd source tree,

Well, a resize_ffs tool.

> Being able to growfs a mounted filesystem would be nice but
> implementing that far exceeds my current knowledge of filesystem
> voodoo.

It also is only part of what you want.  resize_ffs permits enlarging a
filesystem within its partition; it does not even try to do anything
about enlarging the underlying partition.  Nor does it run "live",
though it probably could be made to without too much headache, by
adding something like Auspex's filesystem isolation feature (snapshots
look similar and probably would help).

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