[libvirt] [frankschreuder at outlook.com: Fwd: Reproducible live disk detach bug]

[Laine Stump]

On 08/06/2015 05:00 PM, Richard W.M. Jones wrote:
> Forwarding from Frank Schreuder who was having some issues
> sending this to the mailing list.
>
> ----- Begin forwarded message -----
>
>> Subject: Reproducible live disk detach bug
>>
>> Hey guys,
>>
>> I am currently running into a reproducible libvirt bug when I detach a
>> disk from a running VM using libvirt 1.2.18 and qemu 2.3.0.
>>
>> I can attach a disk to a running VM without any problems.
>> During the attach operation I don't create a disk alias name, which
>> results in libvirt generating an unique alias.
>> Using virsh dumpxml I get the following output(snippet):
>>
>> <disk type='file' device='disk'>
>>       <driver name='qemu' type='raw' cache='none'/>
>>       <source file='/var/local/mnt/test-storage/test-storage.raw'/>
>>       <backingStore/>
>>       <target dev='vde' bus='virtio'/>
>>       <iotune>
>>         <read_bytes_sec>94371840</read_bytes_sec>
>>         <write_bytes_sec>31457280</write_bytes_sec>
>>         <read_iops_sec>1000</read_iops_sec>
>>         <write_iops_sec>200</write_iops_sec>
>>       </iotune>
>>       <alias name='virtio-disk4'/>
>>       <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>
>> </disk>
>>
>> As you can see, libvirt generates 'virtio-disk4' as an alias. But as
>> soon as I try to detach the disk from the running VM, I will receive the
>> following two errors in my libvirt log:
>>
>> 2015-08-06 10:48:59.943+0000: 11657: warning :
>> qemuMonitorJSONHandleDeviceDeleted:939 : missing device in device
>> deleted event
>> 2015-08-06 10:48:59.945+0000: 11662: error :
>> qemuMonitorTextDriveDel:2594 : operation failed: deleting
>> drive-virtio-disk4 drive failed: 2015-08-06T10:48:59.945058Z Device
>> 'drive-virtio-disk4' not found
>>
>> Apparently libvirt tries to detach the disk with a drive- prefix, which
>> doesn't exist?

I can't tell you right off why the failure message is there, but the
alias with the added "drive-" is easily explained.

Each <disk> element in the libvirt config corresponds to two entities in
the qemu virtual machine - a "device" component (what is seen by the
guest) and a "drive" component (what resources are used on the host to
back up that device). Each of these corresponds to a separate qemu
commandline argument (named "-device" and "-drive"), and each of those
must have a different, unique "id" option. For example, this is what is
added to my qemu commandline when I add a virtio disk named
"rawhide.qcow2" to my virtual machine:

-drive
file=/var/lib/libvirt/images/rawhide.qcow2,if=none,id=drive-virtio-disk0,format=qcow2
-device
virtio-blk-pci,scsi=off,bus=pci.2,addr=0x5,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1


If you look at the status XML of the domain while it is running, you'll
see that the <alias> for the disk element is "virtio-disk0", and that's
what is given to the -device half of the disk as its qemu id. The -drive
half uses "drive-$alias" as its qemu id though (you'll notice that the
-device half references that id with its "drive" option - that's how the
two halves are tied together by qemu).

So, there is nothing unusual about qemu wanting to delete something
called drive-virtio-disk4 when you have a <disk> with
alias='virtio-disk4'. It obviously isn't right that an error message is
being logged. Something sounds familiar about the "missing device in
device deleted event" message, but I can't recall details. Does anyone
else have a better memory? Frank - would it be possible for you to try
this same sequence of events with the same libvirt and an older qemu
(and/or vice-versa)?