Re: [PATCH] lib/core/coio_file: Use eio_sendfile_sync instead of a chunk mode

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Fri, Apr 12, 2019 at 04:43:23PM +0300, Vladimir Davydov wrote:
> > +	ssize_t nwritten = eio_sendfile_sync(dest_fd, source_fd, 0, st.st_size);
> > +	if (nwritten != st.st_size) {
> > +		say_syserror("sendfile, [%s -> %s]",
> > +			     fio_filename(source_fd),
> > +			     fio_filename(dest_fd));
> 
> Other coio/fio methods don't log errors so I don't think this one
> should, either.

But fio_ methods which we used before the patch do print an error,
so we should continue printings.

> An extract from the sendfile manual page:
> 
> } sendfile() will transfer at most 0x7ffff000 (2,147,479,552) bytes,
> } returning the number of bytes actually transferred. (This is true on
> } both 32-bit and 64-bit systems.)
> 
> So I don't think using sendfile is equivalent to read+write...

Actaully man page is a bit wrong, at most it may handle up to 0x7fffffff
bytes. And here is a question do we really ever going to transfer such big
files? Moreover using read/write with 4K buffer for big files is pure and
utter crap, the only thing which saves situation a bit is a page cache,
but still such small buffer requires a number of context switches :)

But I agree that using sendfile here actually may trim file. I'll take
a look at evening but I think we should rather use sendfile with offsets
in a cycle. Seriouly, using 4K buffer is the worst possible algo :)