I create a 4x256GB-SSD RAID0(/dev/md1) and I test the performance through fio.

fio config:

ioengine=libaio
direct=1
time_based
runtime=120
ramp_time=30
size=100g

The sequential read/write performance is:

read: 2000MB/swrite: 1800MB/s

Now I make a ext4 on the RAID0(/dev/md1) and mount on /home/.

ANd I create a 100G-disk.qcow2 by guestfish.

$guestfish><fish>: disk-create disk.qcow2 qcow2 100*1024*1024*1024><fish>: run><fish>: mkfs ext4 /dev/sda><fish>: shutdown><fish>: quit

No errors showed.

I mount the disk.qcow2 on the /home/test/, and create a 50GB file.

$dd if=/dev/zero of=/home/test/file bs=1M count=51280

Test sequential read/write again through fio and config almost the same:

ioengine=libaio
direct=1
time_based
runtime=120
ramp_time=30
size=50g

and the testing filename is /home/test/file

and the performance is :

read: 86MB/swrite: 43MB/s

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