JLS2009: A Btrfs update

I understand what you mean now, and that would be a considerable improvement
over serial fsyncs alone. I think you can more or less do the same thing now
on Linux with sync_file_range(...,SYNC_FILE_RANGE_WRITE). Without additional
flags that schedules asynchronous write out of the specified part of the
file. Then when you are all done, call fsync on every fd in the list, as you
say.

That is still somewhat problematic though, since sync_file_range will not
initiate write out of the metadata, which could be significant. Depending on
the way the filesystem handles metadata you could have a very similar
problem, with a journal write and synchronous wait for every fsync...So
something like fadvise options that schedules data and/or metadata for
immediate writeout would be helpful there.