[SPDK] Why Open-CAS-Linux 's performance even surpasses SPDK + OCF ?

1. SPDK + OCF (Using a HDD as AIO bdev, a NVMe SSD as NVMe bdev)

LD_PRELOAD=/home/sankuai/spdk/examples/bdev/fio_plugin/fio_plugin
/usr/local/bin/fio --name=SPDK_OCF --filename=AIO0 --rwmixwrite=30
--rw=randrw --bs=4kb --direct=1 --time_based --size=100% --norandommap
--iodepth=128 --numjobs=4 --ioengine=spdk_bdev --runtime=300
--group_reporting --randrepeat=0 --thread --random_distribution=zipf:1.1
--spdk_conf=/home/sankuai/spdk.conf

Results:
read: IOPS=2129
write: IOPS=912

2. Open-CAS-Linux (process IO by kernel)

/usr/local/bin/fio --name=OPEN_CAS_LINUX --filename=/dev/cas1-1
--rwmixwrite=30 --rw=randrw --bs=4kb --direct=1 --time_based --size=100%
--norandommap --iodepth=128 --numjobs=4 --ioengine=libaio --runtime=3600
--group_reporting --randrepeat=0 --random_distribution=zipf:1.1

Results:
read: IOPS=2950
write: IOPS=1267