[3.0] Use fallocate in incremental mode

[bchess]

Regarding

        if not incremental:
            write_dependency = self._maybe_fallocate(write_specs)

Eta0 last week •
As far as I can tell, this is the only time this flag is used, and it doesn't actually correspond to whether it is an incremental write or not, only whether it is from a call to write_tensor or not, so this flag could be renamed for clarity. That aside, for a large enough single tensor, it's still fine to call fallocate; it can proceed in the background during tensor and metadata preparation, including hash calculation.

@bchess bchess 2 days ago
Yeah, but the logic inside _maybe_fallocate() uses its tensors list to calculate total size. To work in incremental mode it'd need to be updated to account for past writes.

[Eta0]

Copying another of my comments from that thread (#127 (comment)):

fallocate extends the file relative to the current file position, so it doesn't need anything special to account for past writes if the file position is reliable; it can just progressively extend the file.