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

[lampahome]

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:

>
> *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=libaiodirect=1time_basedruntime=120ramp_time=30size=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
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