Re: [PATCH 2/2] xfs: reduce ilock acquisitions in xfs_file_fsync

[Christoph Hellwig <hch@lst.de>]

On Sat, Jan 23, 2021 at 08:08:01AM +1100, Dave Chinner wrote:
> That is, we already elide the call to generic_write_sync() in direct
> IO in the case that the device supports FUA and it's a pure
> overwrite with no dirty metadata on the inode. Hence for a lot of
> storage and AIO/io_uring+DIO w/ O_DSYNC workloads we're already
> eliding this fsync-based lock cycle.
> 
> In the case where we can't do a REQ_FUA IO because it is not
> supported by the device, then don't we really only need a cache
> flush at IO completion rather than the full generic_write_sync()
> call path? That would provide this optimisation to all the
> filesystems using iomap_dio_rw(), not just XFS....
> 
> In fact, I wonder if we need to do anything other than just use
> REQ_FUA unconditionally in iomap for this situation, as the block
> layer will translate REQ_FUA to a write+post-flush if the device
> doesn't support FUA writes directly.
> 
> You're thoughts on that, Christoph?

For the pure overwrite O_DIRECT + O_DSYNC case we'd get away with just
a flush.  And using REQ_FUA will get us there, so it might be worth
a try.