Created attachment 1427604 [details]
libvirtd.log

Description of problem:
The error msg is not clear when blkdeviotune a cdrom without media for a running guest

Version-Release number of selected component (if applicable):
libvirt-3.9.0-14.el7_5.4.x86_64
qemu-kvm-rhev-2.10.0-21.el7_5.2.x86_64


 How reproducible:
 100%

 Steps to Reproduce:
 1.add a cdrom without media to guest xml:
   <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
    <target dev='hda' bus='ide'/>
      <readonly/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

 2.#virsh start 02test

 3.# virsh blkdeviotune 02test hda
error: Unable to get block I/O throttle parameters
error: internal error: block_io_throttle inserted entry was not in expected format

 Actual results:
1. In step3,executed # virsh blkdeviotune 02test hda ,report unclear error.
2. The error behaves differently with a shutoff scenario, refer to addtional info.

 Expected results:
The error should show its specific reason(empty disk or sth), to distinguish with other potential issues such as bz872582.


 Additional info: 
1. when guest shutoff, it gives an output:
# virsh shutdown 02test
# virsh blkdeviotune 02test hda
total_bytes_sec: 0
read_bytes_sec : 0
write_bytes_sec: 0
total_iops_sec : 0
read_iops_sec  : 0
write_iops_sec : 0
total_bytes_sec_max: 0
read_bytes_sec_max: 0
write_bytes_sec_max: 0
total_iops_sec_max: 0
read_iops_sec_max: 0
write_iops_sec_max: 0
size_iops_sec  : 0
group_name     : 
total_bytes_sec_max_length: 0
read_bytes_sec_max_length: 0
write_bytes_sec_max_length: 0
total_iops_sec_max_length: 0
read_iops_sec_max_length: 0
write_iops_sec_max_length: 0

2. when guest running with a media in cdrom, it works well.