Re: [Qemu-devel] Re: RFC v2: blockdev_add & friends, brief rationale, QMP docs

[Kevin Wolf]

Am 16.06.2010 15:41, schrieb Anthony Liguori:
> On 06/16/2010 07:41 AM, Markus Armbruster wrote:
>> Kevin Wolf<address@hidden>  writes:
>>> But it's painful to type for the user. After all -blockdev on the
>>> command line is for the user, as tools should use QMP. Also note that
>>> this syntax mixes format and protocol options on one line which I
>>> consider confusing at best.
>>>
>>> As I told Markus already in private before he posted this, I prefer the
>>> bracket solution for its clarity and simplicity, even though it comes at
>>> the cost of having additional characters that need to be escaped.
>>>      
>> I dont't think 1. is less painful than 3.  Let's compare the two:
>>
>> * Single protocol: identical with suitable syntactical sugar, namely
>>
>>        -blockdev id=blk1,file=fedora.img
>>    
> 
> First, let me say that -blockdev is not something that I believe is 
> targeted at users.  It's incredible unfair for us to expect a user to type:
> 
> -blockdev id=blk1,file=fedora.img -device ide-drive,drive=blk1,bus=0,unit=0
> 
> Instead of:
> 
> -hda fedora.img

Sure thing, as long as -hda provides all the options. I usually start
off with -hda, but after a while I need to set some option and switch to
-drive. This is what most users are using today.

If we're not going to extend -drive to cover all features, then users
will (have to) start using -blockdev.

> I had to look up the device syntax just to write that.  There's no way 
> users are going to do this.  We should drop any notion of syntactical 
> sugar IMHO.  -blockdev is for management tools, scripts, and as an 
> infrastructure for config files.

In that case, let's go for the JSON version. But it requires that
everything that -blockdev provides is accessible with -drive, too (or
that we're okay with users hating us).

> 
>>    Unsugared it's
>>
>>        -blockdev id=blk1,format=raw,protocol=[file,file=fedora.img]
>>    vs.
>>
>>        -blockdev id=blk1,format=raw,protocol=file,file=fedora.img
>>    
> 
> Specifying nesting in a single option is a bad idea.  It should be:
> 
> -blockdev id=blk1,format=raw,protocol=blk2  \
> -blockdev id=blk2,file=fedora.img

I agree, but again, this makes it an option only for tools.

> But honestly, I'm thoroughly confused about the distinction between 
> protocol and format.  I had thought that protocols were a type of format 
> and I'm not sure why we're making a distinction.

Technically, they are mostly the same. Logically, they are not. You have
one image format driver (raw, qcow2, ...) that accesses its image data
through one or more stacked protocols (file, host_device, nbd, http, ...).

In the past we've had quite some trouble because there was no clear
distinction. raw and file was the same. If you had an image on a block
device, you were asking for trouble.

>>    I sure prefer the latter.  The brackets look like noise.  You need to
>>    understand protocol stacking for them to make any sense.
>>
>>    Regarding confusion caused by mixing format and protocol options: yes,
>>    the brackets force you to distinguish between protocol options and
>>    other options.  But I doubt that'll reduce confusion here.  Either you
>>    understand protocols.  Then I doubt you need brackets to unconfuse
>>    you.  Or you don't understand protocols.  Then whether to put an
>>    option inside or outside the brackets is voodoo.
>>    
> 
> If the above is necessary just to create a raw image, then we're doing 
> something wrong in the block layer.  If should be possible to just say:
> 
> -blockdev id=blk1,format=raw,file=fedora.img

I think we all agree on this (although it contradicts what you said
above, because file is a property of the protocol). The question is how
to specify protocols explicitly.

Kevin