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[Vishvananda Ishaya <vishvananda@xxxxxxxxx>]

The limitation is due to what is supported by the qemu-img snapshot -c command
AFAIK this works with qcow only, but perhaps other formats are supported.

use_cow_image=true # always works

use_cow_image=false # only works if force_raw_images=false AND original_image_format = qcow 

Vish

On Apr 10, 2012, at 10:42 AM, Lorin Hochstein wrote:

> Vish:
> 
> For documentation purposes, if the user wants to be able to do snapshots, what combinations of the following three variables are allowed?
> 
> 1. original image format (qcow2 | raw)
> 2. use_cow_image flag (true | false)
> 3. force_raw_images flag (true | false)
> 
> 
> Take care,
> 
> Lorin
> --
> Lorin Hochstein
> Lead Architect - Cloud Services
> Nimbis Services, Inc.
> www.nimbisservices.com
> 
> 
> 
> 
> 
> On Apr 10, 2012, at 1:32 AM, Vishvananda Ishaya wrote:
> 
>> 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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