> Specifying a big blocksize doesn't mean the transfer will be faster.
> send/sendfile won't send more than a certain amount of bytes anyways.

The transfer won't be faster mainly because it's really I/O bound.
But it will use less CPU, only because you're making less syscalls.

> If I'm not mistaken I recall from previous benchmarks that after a certain point (131072 or something) increasing the blocksize results in equal or even worse performances.

I can perfectly believe this for a send loop, maybe because you're
exceeding the socket buffer size, or because your working set doesn't
fit into caches anymore, etc.
But for sendfile(), I don't see how calling it repeatedly could not be
slower than calling it once with the overall size: that's how netperf
and vsftpd use it, and probably others.

> Another thing I don't like is that by doing so you implicitly assume that the file is "fstat-eable". I don't know if there are cases where it's not, but the less assumptions we do the better.

Well, the file must be mmap-able, so I doubt fstat() is the biggest concern...