[libvirt] New QEMU daemon for persistent reservations

[Michal Privoznik]

On 11/26/2017 09:32 AM, Peter Krempa wrote:
> On Fri, Nov 24, 2017 at 15:38:54 +0100, Michal Privoznik wrote:
>> On 08/22/2017 06:27 PM, Paolo Bonzini wrote:
>>> Hi all,
>>>
>>
>> Sorry for resurrecting old thread but seems like there was no agreement
>> reached.
>>
>> We don't want to expose any paths because the fact that PR helper is a
>> separate binary that uses a UNIX socket to talk to qemu is a
>> implementation detail of qemu. Other HVs may have it differently.
>>
>> However, we want to be able to turn it on/off or not mention it at all
>> on per-disk basis. So what has been suggested so far is:
>>
>>   <disk type='block' device='disk'>
>>     <source dev='/dev/sda'>
>>     <target dev='sda' bus='scsi'/>
>>     <reservations enable='yes'/>
>>   </disk>
> 
> Note that if the reservation is applied to the image and not the disk
> the element should become child of the <source> elememt.

> Especially if
> there is only a ever so slight chance that it will work with formats
> supporting backing store (qcow2) and thus there would be a need to
> configure it per-image.

The way I understand PRs is that they are format agnostic, so there's
not (nor there will be) per image configuration. Nevertheless,
<reservations/> concern disk image so I will place the element under
<source/>. Good point.

What is still unclear is what should happen when the helper daemon dies
while domain is running. I was thinking about this during the weekend
and realized that if we go with the event delivered to libvirtd, we also
need a monitor command to query the state (we need to cover case when
event is fired but daemon is not runnning, it has to query the state
once started).

Michal