Re: Plans to bring QMP 'x-blockdev-reopen' out of experimental?

[Alberto Garcia]

On Wed 02 Dec 2020 06:51:21 PM CET, Kevin Wolf wrote:
>> I had tried this already and it does work when inserting the filter (we
>> know that 'hd0-file' is about to be detached from the parent so we can
>> put it in the list) but I don't think it's so easy if we want to remove
>> the filter, i.e.
>> 
>>    hd0 -> throttle -> hd0-file     ======>     hd0 -> hd0-file
>> 
>> In this case we get a similar error, we want to make hd0-file a child of
>> hd0 but it is being used by the throttle filter.
>> 
>> Telling bdrv_check_update_perm() to ignore hd0's current child
>> (throttle) won't solve the problem.
>
> Isn't this the very same case as removing e.g. a mirror filter from the
> chain? I'm sure we have already solved this somewhere.
>
> Hm, no, it might actually be different in that the throttle node
> survives this, so we do have to check that the resulting graph is
> valid. Do we need a combined operation to remove the throttle node
> from the graph and immediately delete it?

What kind of API are you thinking about?

One basic problem with inserting filter nodes (as opposed to replacing a
node with a different one) is that we have a protocol BDS used twice at
the same time, e.g.

  hd0 -> hd0-file
  throttle -> hd0-file

And then we would reopen hd0 and do the change, but ideally one would
prefer to avoid having hd0-file twice.

There's also the x-blockdev-change function (currently for Quorum only),
I wonder if it could be a better fit for this use case.

Berto