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

[Markus Armbruster]

Kevin Wolf <address@hidden> 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.

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.