Re: [Qemu-devel] [PATCH 0/3] blockdev: Add read-only option to change-blockdev

[Max Reitz <mreitz@redhat.com>]

On 2014-11-28 at 16:43, Markus Armbruster wrote:
> Kevin Wolf <kwolf@redhat.com> writes:
>
>> Am 20.11.2014 um 13:44 hat Max Reitz geschrieben:
>>> The 'change' QMP and HMP command allows replacing the medium in drives
>>> which support this, e.g. floppy disk drives. For some drives, the medium
>>> carries information about whether it can be written to or not (again,
>>> floppy drives). Therefore, it should be possible to change the read-only
>>> state of block devices when changing the loaded medium.
>>>
>>> This series adds an optional additional parameter to the 'change' QMP
>>> and HMP command which allows changing the read-only state in four ways:
>>>
>>> - 'retain': Just keep the status as it was before; this is the current
>>>    behavior and thus this will be the default.
>>> - 'ro': Force read-only access
>>> - 'rw': Force writable access
>>> - 'auto': This opens the new file R/W first, if that fails, the file is
>>>    opened read-only.
>> Not sure if 'auto' is worth implementing (it's a typical HMP default
>> action that no QMP client would use, except that it isn't even the
>> default for HMP), but the implementation looks correct at least.
> QMP eschews magic.  I'd prefer to keep 'auto' out.
>
> HMP can offer it regardless, if it's useful.  But I doubt it will be.
> Few users will need to control this, and fewer will realize they can by
> giving an extra argument.

Well, Kevin made the good point of the user generally knowing whether a 
file should be written to or not. Furthermore, I don't think it would be 
too hard to use rw, then see that access was denied, and then use ro, 
manually. Maybe that's even better than auto, somehow.

Max

> I wish I could tell you to leave change alone because it's a legacy
> dungpile.  Alas, it's still only a dungpile.  I hope we can create a set
> of sane media control commands relatively soon now.