Re: [Qemu-devel] Using the one disk image file on 2 virtual machines at the same time

[Christopher Covington]

On 07/29/2015 01:46 PM, John Snow wrote:
> 
> 
> On 07/29/2015 01:29 PM, Manjong Han wrote:
>> Thanks, Stefan.
>>
>> 2015-07-29 17:46 GMT+09:00 Stefan Hajnoczi <address@hidden>:
>>>
>>> You should probably use qcow2 backing files instead:
>>>
>>>   10G.qcow2 <-- vm001.qcow2
>>>             ^-- vm002.qcow2
>>>
>>> The command to create these files is:
>>>
>>>   qemu-img create -f qcow2 -o backing_file=10G.qcow2 vm001.qcow2.
>>>
>>> Both VMs share the data in 10G.qcow2.  All writes go to vm001.qcow2 or
>>> vm002.qcow2, respectively, so they don't corrupt each other.
>>>
>>
>> I tried to create a backing files, using the commands which you told.
>>
>> $ qemu-img create -f qcow2 -o backing_file=10G.qcow2 vm001.qcow2
>> $ qemu-img create -f qcow2 -o backing_file=10G.qcow2 vm002.qcow2
>>
>> And, I used these backing files on each virtual machines.
>> But, new files weren't written on original disk image(10G.qcow2)..
>> The backing files were working each other.
>>
>>> Standard file systems (ext4, xfs) and volume managers (LVM) are not
>>> cluster-aware by default.  They must only be accessed from one machine
>>> at a time.  Otherwise you risk data corruption.
>>>
>>
>> I think that I must probably use a shared file system like NFS..
>>
> 
> Yes, any files written using the backing files like outlined above will
> put new files in the overlays (e.g. vm001.qcow2 or vm002.qcow2) and NOT
> into the backing file (10G.qcow2)
> 
> this is a safe way to share a base image for an OS, but it's not a
> method of accomplishing a concurrent fileshare.
> 
> You'll want to configure an NFS or CIFS share (etc) in the base image
> and then allow the multiple VMs to utilize that share.

Using a VirtIO-9P based passthrough filesystem on both might produce the same
result with less host-side configuration required.

http://wiki.qemu.org/Documentation/9psetup

Chris

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