bug#9500: [PATCH]: use posix_fallocate where supported

[Paul Eggert]

My read of the situation is that the filesystem guys have
spent a lot of time optimizing ordinary write but they
haven't gotten around to optimizing fallocate because it's
so rarely used -- which means that if one uses fallocate
one gets lousy performance.

It's a chicken and egg problem.

If coreutils started using fallocate now, one can be pretty
sure they'd tune their filesystems over the next few years,
to make fallocate compatible with delayed-write optimizations.
On the other hand if nobody uses fallocate, there will be little
incentive on their part to make it go fast.

It's a question of whether we want to inflict temporary pain
on users for a long-term benefit (early warning of file system
full, which is something I'd dearly love to have).