[Libguestfs] read/write performance through mount point by guestmount

I create a *4x256GB-SSD RAID0(/dev/md1)* and I test the performance through fio. fio config: ioengine=libaiodirect=1time_basedruntime=120ramp_time=30size=100g The sequential read/write performance is: Now I make a ext4 on the RAID0(/dev/md1) and mount on /home/. ANd I create a 100G-disk.qcow2 by guestfish. No errors showed. I mount the disk.qcow2 on the /home/test/, and create a 50GB file. Test sequential read/write again through fio and config almost the same: ioengine=libaiodirect=1time_basedruntime=120ramp_time=30size=50g and the testing filename is /home/test/file and the performance is : how come the performance is so bad? almost down 95%. My machine is: 4.4.0-83-generic, Ubuntu 16.04, x86_64 GNU/Linux