Re: [PATCH for 5.2 1/1] qemu-io: add -V flag for read sub-command

[Denis V. Lunev]

On 8/10/20 3:52 PM, Richard W.M. Jones wrote:
> On Mon, Aug 10, 2020 at 03:35:55PM +0300, Denis V. Lunev wrote:
>> The problem this patch is trying to address is libguestfs behavior on the
>> appliance startup. It starts supporting to use root=UUID definition in
>> the kernel command line of its root filesystem using
>>     file --  /usr/lib64/guestfs/appliance/root
>> This works fine with RAW image, but we are using QCOW2 as a storage to
>> save a bit of file space and in this case we get
>>     QEMU QCOW Image (v3), 1610612736 bytes
>> instead of UUID of the root filesystem.
>>
>> The solution is very simple - we should dump first 256k of the image file
>> like the follows
>>     qemu-io -c "read -V 0 256k" appliance | file -
>> which will provide correct result for all possible types of the appliance
>> storage.
>>
>> Unfortunately, additional option for qemu-io is the only and the simplest
>> solution as '-v' creates very specific output, which requires to be
>> parsed. 'qemu-img dd of=/dev/stdout' does not work and the fix would be
>> much more intrusive.
> I like the idea of the flag - we could also use it in the nbdkit test
> suite.
>
> I wonder if the actual flag ('V') is a good idea because I would
> normally associate that with getting the version of a command.  But
> as these are subcommand flags, that's probably not too relevant here.
we could use '-r' aka --raw and apply it only if -v present.

I think that some opinion from Kevin or Max would be helpful
here.

Den