[CDTS] Disk I/O stress test fail due to returned non-zero value

[Summary]
Disk I/O stress test fail due to returned non-zero value.
Snap info:
Name Version Rev Tracking Publisher Notes
cdts 0.6 87 20.04/stable ce-certification-qa classic
checkbox20 1.20 428 latest/stable ce-certification-qa -

==== checkbox log ====
Testing disk sda
identified as a disk...
test will be run on partition /dev/sda1
/dev/sda1 already mounted at /mnt
953636.625MB of free space avaialble
Forcing memory setting to 8000MB
+ bonnie++ -d /mnt -u root -r 8000
Using uid:0, gid:0.
Writing a byte at a time...done
Writing intelligently...Can't write block.: No such file or directory
Can't write block 524287.
Traceback (most recent call last):
  File "/tmp/nest-q47d8rlh.797b90e3ad83940cf0c5de2249b50fcea8701864e916e7a3ce2dae7792eabc54/storage_test.py", line 146, in <module>
    main()
  File "/tmp/nest-q47d8rlh.797b90e3ad83940cf0c5de2249b50fcea8701864e916e7a3ce2dae7792eabc54/storage_test.py", line 142, in main
    disk_test(udev_name)
  File "/tmp/nest-q47d8rlh.797b90e3ad83940cf0c5de2249b50fcea8701864e916e7a3ce2dae7792eabc54/storage_test.py", line 131, in disk_test
    run_bonnie(mount_dir)
  File "/tmp/nest-q47d8rlh.797b90e3ad83940cf0c5de2249b50fcea8701864e916e7a3ce2dae7792eabc54/storage_test.py", line 88, in run_bonnie
    sp.check_call(cmd, shell=True)
  File "/snap/checkbox20/current/usr/lib/python3.8/subprocess.py", line 364, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command 'bonnie++ -d /mnt -u root -r 8000' returned non-zero exit status 1.

==== error log for command ====
ubuntu@ubuntu:~$ bonnie++ -d /mnt -u root -r 8000
Can't set gid to 0.

ubuntu@ubuntu:~$ sudo cat /etc/group
root:x:0:
daemon:x:1:

[Steps to reproduce]
1.Boot into system
2.Run "disk/storage_device_sda" case

[Expected result]
Test case should finish without error.

[Actual result]
Test case fail due to some unexpected error.

[Failure rate]
100%