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Message-ID: <4922D0AB.3060004@codemonkey.ws>
Date: Tue, 18 Nov 2008 08:26:51 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] add file: migration support (r2)
References: <20081118030437.GA940@shareable.org>
	<200811181257.42351.paul@codesourcery.com>
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To: qemu-devel@nongnu.org
Cc: Charles Duffy <Charles_Duffy@messageone.com>

Paul Brook wrote:
> On Tuesday 18 November 2008, Jamie Lokier wrote:
>   
>> Paul Brook wrote:
>>     
>>>> I'm not sure if "live" migration to a file is really useful.  "Dead"
>>>> migration is useful for suspending a VM to disk.
>>>>         
>>> Right. That's the point I was trying to make.
>>>       
>> I thought of a use for live migration to and from a file.  It's when
>> you have to restart a host while minimising downtime of all its
>> guests.  The liveness reduces that downtime.
>>     
>
> I wouldn't be surprised if this actually took longer, because you have to read 
> more data in when you do the restore.
>   

file: migration is essentially live checkpointing.  It takes longer to 
save but the key is that you do the checkpointing without incurring down 
time.

The real use case would be live migrating to a file (or BDRV) and then 
immediately resuming the source VM after the migration has completed.  
The effect would be live checkpointing.

But without an implementation of file: or bdrv: that are actually live, 
it really is just an academic conversation :-)

Regards,

Anthony Liguori

> Paul
>
>
>