btrfs: use IOMAP_DIO_BOUNCE flag instead of falling back to buffered IO

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>
Signed-off-by: David Sterba <dsterba@suse.com>

0 files changed, 0 insertions, 0 deletions