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[Vishvananda Ishaya <vishvananda@xxxxxxxxx>]

You can disable using backing files with the following config:
use_cow_images=false

You should be aware that you likely won't be able to snapshot images unless you make sure to upload them all in qcow format and also set:
force_raw_images=false

On Apr 9, 2012, at 9:37 PM, William Herry wrote:

> Hi
> 
> I read from an article that said use preallocation can improve disk I/O performance in kvm, when I add it to openstack, suck error come to me
> 
> (nova.rpc.amqp): TRACE: Stderr: 'Backing file and preallocation cannot be used at the same time\nqemu-img: /usr/local/lib/python2.7/dist-packages/nova-2012.1-py2.7.egg/instances/instance-0000000e/disk: error while creating qcow2: Invalid argument\n'
> 
> I was added it to utils.py in virt/libvirt directory (line 77)
> 
> def create_cow_image(backing_file, path):
>     """Create COW image
> 
>     Creates a COW image with the given backing file
> 
>     :param backing_file: Existing image on which to base the COW image
>     :param path: Desired location of the COW image
>     """
>     execute(FLAGS.qemu_img, 'create', '-f', 'qcow2', '-o',
>              'preallocation=metadata,cluster_size=2M,backing_file=%s' %
>               backing_file, path)
> 
> here is the article: http://itscblog.tamu.edu/improve-disk-io-performance-in-kvm/
> 
> so what is Backing file for, can I disable it for use preallocation cause I can't got both
> 
> Thanks
> 
> -- 
> 
> ===========================
> William Herry
> 
> WilliamHerryChina@xxxxxxxxx
> 
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