Re: Do I have to fsync after aio_write finishes (with fallocate preallocation) ?

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Nov 29, 2022 at 11:20:05AM -0800, Shawn wrote:
> Hello all,
> I implemented a write workload by sequentially appending to the file
> end using libaio aio_write in O_DIRECT mode (with proper offset and
> buffer address alignment).  When I reach a 1MB boundary I call
> fallocate() to extend the file.
> 
> I need to protect the write from various failures such as disk unplug
> / power failure.  The bottom line is,  once I ack a write-complete,
> the user must be able to read it back later after a disk/power failure
> and recovery.
> 
> In my understanding,  fallocate() will preallocate disk space for the
> file,  and I can call fsync to make sure the file metadata about this
> new space is persisted when fallocate returns.  Once aio_write returns
> the data is in the disk.  So it seems I don't need fsync after
> aio-write completion, because (1) the data is in disk,  and (2) the
> file metadata to address the disk blocks is in disk.
> 
> On the other hand, it seems XFS always does a delayed allocation
> which might break my assumption that file=>disk space mapping is
> persisted by fallocate.
> 
> I can improve the data-in-disk format to carry proper header/footer to
> detect a broken write when scanning the file after a disk/power
> failure.
> 
> Given all those above,  do I still need a fsync() after aio_write
> completion in XFS to protect data persistence?

Yes.  The only time you don't is if you're performing an O_SYNC write
to a part of a file that you've already written (and fsync'd) that's
entirely below EOF and you've arranged that the filesystem will never
COW or otherwise require metadata updates.

Hey, at least aio_fsync works now...

--D

> Thanks all for your input!
> 
> regards,
> Shawn