Re: [Qemu-devel] [PATCH 0/3] vpc: support probing of fixed size images

[Eric Blake]

On 08/15/2014 08:10 AM, Jeff Cody wrote:
> On Fri, Aug 15, 2014 at 08:00:19AM -0600, Eric Blake wrote:
>> On 08/15/2014 07:37 AM, Kevin Wolf wrote:
>>
>>> We can choose Markus's suggestion of using the file name to guess the
>>> format. I don't really like it much, but it seems like a fair compromise
>>> that doesn't hurt usability as much.
>>
>> In other words, if a user gives a file a "known suffix", then it is
>> their own fault if they made that file raw and the guest then happened
>> to convert the file to the format matching the suffix?  Or would this
>> start giving warnings if the known suffix doesn't match the probed contents?
>>
> 
> (Eric, I should have cc'ed you on my last email, sorry)
> 
> Image this scenario:
> 
>     existing chain created a while ago, via:
> 
>         qemu-img create -f qcow2 foo.img 1G
>         qemu-img create -f qcow2 -b foo.img bar.img 1G
> 

Libvirt refuses to create files without backing format encoded; but is
still stuck dealing with files created externally (such as qemu-img
create) where it was forgotten.

> 
> User launches qemu by this commandline:
> 
>     qemu-kvm -drive file=bar.img,format=qcow2
> 
> 
> Old behavior:
> 
>   | foo.img | <--- | bar.img |
>   | (qcow2) |      | (qcow2) |
> 
> New behavior:
> 
>   | foo.img | <--- | bar.img |
>   | (raw)   |      | (qcow2) |

Libvirt already assumed raw for any image where encoding was not
specified, so this scenario is already a risky one for the user.

> 
> 
> So I think we want to make sure that we don't just fall back to raw
> for unknown filename extensions.

Remember, if a probe determines a file is raw, it is safe to use as raw.
 Falling back to raw is IDEAL.

What _might_ be nice is teaching qemu-img to auto-encode the backing
format during 'qemu-img create' (it can be assumed that _at creation
time_, the backing file hasn't had a chance to be corrupted by the guest
yet) rather than the current behavior of only encoding an explicit
format given by the user.  Furthermore, an explicit encoding from the
user should ALWAYS be trusted, and the user can override any place where
automatic heuristics would have guessed differently (whether those
heuristics were based on probing, file extension, or otherwise).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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