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Date: Wed, 05 Nov 2008 08:09:56 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Live migration - exec: support to be reintroduced?
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To: qemu-devel@nongnu.org

Chris Lalancette wrote:
> Anthony Liguori wrote:
>   
>> Charles Duffy wrote:
>>     
>>> Howdy.
>>>
>>> KVM's live migration support used to support an exec: protoco, 
>>> allowing  either a completely arbitrary transport or a mechanism for 
>>> storing system state to a separate file for later resurrection, being 
>>> used in the latter by libvirt's qemu driver. I notice that this 
>>> support no longer exists in the current codebase.
>>> Would a patch reimplementing this support be welcome?
>>>       
>> Absolutely.  I wasn't aware that libvirt used the exec: protocol.  I'm 
>> somewhat surprised by that.  What did it use it for?
>>     
>
> Hm, I didn't realize it used it either, but it uses it for doing a save operation:
>
> static int qemudDomainSave(virDomainPtr dom,
>                            const char *path) {
> ...
>     if (asprintf (&command, "migrate \"exec:"
>                   "dd of='%s' oflag=append conv=notrunc 2>/dev/null"
>                   "\"", safe_path) == -1) {
>         qemudReportError(dom->conn, dom, NULL, VIR_ERR_OPERATION_FAILED,
>                          "%s", _("out of memory"));
>         VIR_FREE(safe_path);
>         return -1;
>     }
>     free(safe_path);
>
>     if (qemudMonitorCommand(driver, vm, command, &info) < 0) {
> ...
>
> I don't know if there is a better way to do that with the qemu monitor, to avoid
>  using migrate altogether.  In any case, I'm sure other people would find exec:
> useful as well.
>   

Sure.  It would make sense for it to use file: instead of exec: though.

However, why is it not just using savevm?  Saving the CPU state in the 
absence of checkpointing storage doesn't make a lot of sense.

Regards,

Anthony Liguori