Re: [Qemu-devel] [PATCH 2/2] Documentation: Update image format information

[Kevin Wolf]

Am 22.11.2012 08:37, schrieb Stefan Hajnoczi:
> On Wed, Nov 21, 2012 at 04:22:07PM +0100, Kevin Wolf wrote:
>> Am 21.11.2012 16:14, schrieb Stefan Hajnoczi:
>>> On Wed, Nov 21, 2012 at 02:23:57PM +0100, Kevin Wolf wrote:
>>>>  @item qed
>>>> -Image format with support for backing files and compact image files (when 
>>>> your
>>>> -filesystem or transport medium does not support holes).  Good performance 
>>>> due
>>>> -to less metadata than the more featureful qcow2 format, especially with
>>>> -cache=writethrough or cache=directsync.  Consider using qcow2 which will 
>>>> soon
>>>> -have a similar optimization and is most actively developed.
>>>> +Old QEMU image format. Left for compatibility.
>>>> +
>>>> +For new images, use qcow2 instead. You might want to consider using the
>>>> address@hidden option to get a more QED-like behaviour.
>>>
>>> The first sentence should be kept, it describes the general feature set
>>> and scope of this image format.  I agree that the rest of the paragraph
>>> can be dropped.
>>>
>>> You could insert a statement saying that qcow2 is now preferred because
>>> it is actively developed and offers advanced features and performance as
>>> the very first sentence.
>>
>> It's the same terse description as for qcow1. If we decided to describe
>> the format in more detail, we should do so consistently, and probably
>> also for non-native formats. (But why? The only use case is
>> compatibility with other/older hypervisors, which is mentioned.)
> 
> Yes, we should have descriptions of other formats too.
> 
> Users dealing with existing guests using these formats should still have
> access to general information about the formats.  For example, if I'm a
> new user and it's my job to work with an existing qcow1 disk image then
> it's not very helpful to just see a terse "Use qcow2 instead" message.
>
> It's not good to drop documentation on a feature because it is
> deprecated.

I can see your point, but the whole section starts to become a bit
lengthy then for a man page. Also, I don't think that your qcow1 user
really needs a detailed description - he already has a format and
doesn't have to choose one, so the characteristics aren't that
important. If he considers converting the image, we're back to raw and
qcow2.

Maybe we should keep the current descriptions for raw and qcow2 (as they
are what users really choose between for new images), and extend and
move the documentation of other formats into qemu-doc.texi and mention
them in the man page of qemu-img only as "Also supported for
compatibility with other hypervisors or older QEMU versions: vmdk, vdi,
qed, ... See the QEMU Emulator User Documentation for more information
on these."

Kevin