Re: [Qemu-devel] Re: [patch 2/3] Add support for live block copy

[Avi Kivity <avi@redhat.com>]

On 02/22/2011 11:11 PM, Anthony Liguori wrote:
>> The commit file is considered reliable storage for the result of image
>> switch operation. Think of the following scenario:
>>
>> - mgmt app requests live copy of guests ide1-hd0
>> from /a/image.img to /b/image.img.
>> - mgmt app dies.
>> - guest switches to /b/image.img, /a/image.img is outdated.
>> - guest dies.
>>
>> Notifying the switch via QMP would not be reliable in this case.
>
> But this is true of any number of operations in QEMU such as hotplug 
> where if a management tool dies after requesting hotplug and then the 
> guest dies before the management tool reconnects, it's never known 
> whether it's truly connected or not.
>
> The only way to robustly solve this is for QEMU to maintain a stateful 
> config file.

Or, the management tool can remember that it tries to hotplug, 
reconnect, query qemu (or just retry), and proceed according to the result:

- insert record in config for the new device, status = inactive
- commit
- tell qemu to hotplug
<crash>
- restart, read database, see inactive device
- query qemu
- if device is there, mark as active, commit
- else, tell qemu to hotplug, when successful, mark as active, commit


>   A one-off for this particular command doesn't seem wise to me.

Strongly agreed.

-- 
error compiling committee.c: too many arguments to function