Re: Vectored I/O in bulk_write.c

I canvassed Andres off-list since smgrzeroextend() is his invention,
and he wondered if it was a good idea to blur the distinction between
the different zero-extension strategies like that. Good question. My
take is that it's fine:

mdzeroextend() already uses fallocate() only for nblocks > 8, but
otherwise writes out zeroes, because that was seen to interact better
with file system heuristics on common systems. That preserves current
behaviour for callers of plain-old sgmrextend(), which becomes a
wrapper for smgrwrite(..., 1, _ZERO | _EXTEND). If some hypothetical
future caller wants to be able to call smgrwritev(..., NULL, 9 blocks,
_ZERO | _EXTEND) directly without triggering the fallocate() strategy
for some well researched reason, we could add a new flag to say so, ie
adjust that gating.

In other words, we have already blurred the semantics. To me, it
seems nicer to have a single high level interface for the same logical
operation, and flags to select strategies more explicitly if that is
eventually necessary.