Re: [PATCH v4 3/3] btrfs: use IOMAP_DIO_BOUNCE flag instead of falling back to buffered IO

[David Sterba <dsterba@suse.cz>]

On Tue, Jun 16, 2026 at 05:42:37PM +0930, Qu Wenruo wrote:
> Previously btrfs forces direct writes to fall back to buffered ones if the
> inode has data checksum or the profile has duplication.
> 
> That fallback is to avoid the content being modified that the final
> content may mismatch with the checksum or the other mirrors.
> 
> That brings a pretty huge performance cost, which already caused some
> concern at that time.
> 
> But later upstream commit c9d114846b38 ("iomap: add a flag to bounce
> buffer direct I/O") introduced a new method by copying the content into
> new pages, and do all the operations based on the newly allocated pages.
> 
> So let btrfs to utilize the new flag for direct writes if we require
> stable folios.
> 
> There is a quick benchmark, using the following fio setup:
> 
>  fio --name=randwrite --filename $mnt/foobar --ioengine=libaio --size=4G \
>      --rw=randwrite --iodepth=64 --runtime=60 --time_based --direct=1 \
>      --bs=$blocksize
> 
> Unit is MiB/s.
> 
>  Blocksize | Zero-copy (*) | Buffered |   Bounce
> -----------+---------------+----------+-----------
>         4K |          35.1 |     17.1 |      33.8
>        64K |           522 |      251 |       492
> 
> *: This is done by reverting the commit 968f19c5b1b7 ("btrfs: always
>    fallback to buffered write if the inode requires checksum")
> 
> Although with page bouncing the performance is only around 95% of
> true-zero copy, it's still almost double the performance of buffered
> fallback.
> 
> There will be a small change in behavior, since we're using
> IOMAP_DIO_BOUNCE flag to allocate new folios, NOWAIT flag will
> immediately fail.
> 
> So for true NOWAIT direct IOs, NODATASUM and RAID0/SINGLE profiles are
> still required.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

The block layer patches have been merged and our for-next is now based
on 7.2-rc1 so pleaase add this one too so we can get back the dio
performance. Thanks.