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

[John Snow <jsnow@redhat.com>]

On 07/29/2015 01:29 PM, Manjong Han wrote:
> Thanks, Stefan.
> 
> 2015-07-29 17:46 GMT+09:00 Stefan Hajnoczi <stefanha@gmail.com>:
>>
>> 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.

--js