Re: [Qemu-block] [Qemu-devel] [PATCH 7/7] iotests: Add test for image creation fallback

[Eric Blake]

On 7/15/19 4:48 AM, Max Reitz wrote:

>>> +
>>> +_supported_fmt raw
>>
>> Why is this stating "raw" here...
>>
>>> +_supported_proto nbd
> 
> Because it’s an NBD test.
> 
>>> +_supported_os Linux
>>> +
>>> +
>>> +_make_test_img 64M
> 
> Also, because I don‘t want this to create a qcow2 image.  This should
> just set up a raw NBD node.
> 
>>> +echo
>>> +echo '--- Testing creation ---'
>>> +
>>> +$QEMU_IMG create -f qcow2 "$TEST_IMG" 64M | _filter_img_create
>>
>> ... and using qcow2 here instead?
> 
> Practical answer: Nobody tests qcow2+nbd.  Ever.  Because it is
> generally a stupid combination.  We need it for this test, though,
> because NBD is the simplest way to get a fixed-size block device.

There are definitely some broken things if you try qcow2+nbd. However, I
someday hope to implement a proposed NBD_CMD_RESIZE extension to the
protocol, at which point, it will be a lot easier to run qcow2+nbd
(where qcow2 can then advantage of automatic resizes of the protocol
layer, the same as it does for regular files), so part of that effort
may be figuring out how to make iotests cleanly support qcow2+nbd in
more situations.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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