Re: [PATCH 8/8] iomap: Add writethrough for O_SYNC

[Matthew Wilcox <willy@infradead.org>]

On Thu, Aug 12, 2021 at 02:16:03PM +0100, Christoph Hellwig wrote:
> On Wed, Aug 11, 2021 at 03:46:47AM +0100, Matthew Wilcox (Oracle) wrote:
> > For O_SYNC writes, if the filesystem has already allocated blocks for
> > the range, we can avoid marking the page as dirty and skip straight to
> > marking the page as writeback.
> 
> So this just optimizes O_SYNC overwrites.  How common are those for
> bufered I/O?  I know databases use them a lot with direct I/O, but for
> buffere I/O this seems like an odd I/O pattern.

As the comment says:

+       /* Can't allocate blocks here because we don't have ->prepare_ioend */

Give me a way to allocate blocks and it can do better!  I didn't realise
this was going to be a problem when I embarked on this, but attempting
to do IO to wild addresses made me realise that most appending O_SYNC
writes are IOMAP_DELALLOC and so don't have allocated blocks.

And it's not just overwrites.  If you open(O_SYNC|O_TRUNC) and then
write ten bytes at a time, the first write to each block will cause us
to fall back to writeback pages, but subsequent writes to a block will
writethrough.