Btrfs currently limits direct I/O reads to a single sector, which goes
back to commit c329861d ("Btrfs: don't allocate a separate csums
array for direct reads") from Josef.  That commit changes the direct I/O
code to ".. use the private part of the io_tree for our csums.", but ten
years later that isn't how checksums for direct reads work, instead they
use a csums allocation on a per-btrfs_dio_private basis (which have their
own performance problem for small I/O, but that will be addressed later).

There is no fundamental limit in btrfs itself to limit the I/O size
except for the size of the checksum array that scales linearly with
the number of sectors in an I/O.  Pick a somewhat arbitrary limit of
256 limits, which matches what the buffered reads typically see as
the upper limit as the limit for direct I/O as well.

This significantly improves direct read performance.  For example a fio
run doing 1 MiB aio reads with a queue depth of 1 roughly triples the
throughput:

Baseline:

READ: bw=65.3MiB/s (68.5MB/s), 65.3MiB/s-65.3MiB/s (68.5MB/s-68.5MB/s), io=19.1GiB (20.6GB), run=300013-300013msec

With this patch:

READ: bw=196MiB/s (206MB/s), 196MiB/s-196MiB/s (206MB/s-206MB/s), io=57.5GiB (61.7GB), run=300006-300006msc
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>