Re: [rfc] fsync_range?

[Jamie Lokier <jamie@shareable.org>]

Nick Piggin wrote:
> Just wondering if we should add an fsync_range syscall like AIX and
> some BSDs have? It's pretty simple for the pagecache since it
> already implements the full sync with range syncs anyway. For
> filesystems and user programs, I imagine it is a bit easier to
> convert to fsync_range from fsync rather than use the sync_file_range
> syscall.
> 
> Having a flags argument is nice, but AIX seems to use O_SYNC as a
> flag, I wonder if we should follow?

I like the idea.  It's much easier to understand than sync_file_range,
whose man page doesn't really explain how to use it correctly.

But how is fsync_range different from the sync_file_range syscall with
all its flags set?

For database writes, you typically write a bunch of stuff in various
regions of a big file (or multiple files), then ideally fdatasync
some/all of the written ranges - with writes committed to disk in the
best order determined by the OS and I/O scheduler.

For this, taking a vector of multiple ranges would be nice.
Alternatively, issuing parallel fsync_range calls from multiple
threads would approximate the same thing - if (big if) they aren't
serialised by the kernel.

-- Jamie