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

[Alberto Garcia]

On Tue 20 Oct 2020 10:23:33 AM CEST, Kevin Wolf wrote:
>> I forgot to add, we still don't support changing bs->file with this
>> command, so I guess that would be one blocker?
>> 
>> There's no other way of inserting filter nodes, or is there?
>
> Not that I'm aware of.
>
> So yes, changing bs->file is the one thing I had in mind for
> implementing before we mark it stable.
>
> I'm not entirely sure if we should make some restrictions or allow
> arbitrary changes. If it's only about filters, we could check that the
> node returned by bdrv_skip_filters() stays the same. This would be
> almost certainly safe (if the chain is not frozen, of course).
>
> If people want to dynamically insert non-filters (maybe quorum?), it
> might be more restrictive than necessary, though.
>
> Other cases like inserting a qcow2 file in the chain where the old
> child becomes the backing file of the newly inserted node should in
> theory already be covered by blockdev-snapshot.

Hi,

I have been working a bit on this and I have doubts about the
following situation: let's say we have a normal qcow2 image with two
BDS for format (node-name "hd0") and protocol ("hd0-file"):

   hd0 -> hd0-file

{ "execute": "blockdev-add", "arguments":
   {'driver': 'file', 'node-name': 'hd0-file', 'filename':  'hd0.qcow2 }}
{ "execute": "blockdev-add", "arguments":
   {'driver': 'qcow2', 'node-name': 'hd0', 'file': 'hd0-file'}}

Now we want to use x-blockdev-reopen to insert a throttle filter
between them, so the result would be:

   hd0 -> throttle -> hd0-file

First we add the filter:

{ "execute": "object-add", "arguments":
   { 'qom-type': 'throttle-group', 'id': 'group0',
     'props': { 'limits': { 'iops-total': 1000 } } } }
{ "execute": "blockdev-add", "arguments":
   { 'driver': 'throttle', 'node-name': 'throttle0',
     'throttle-group': 'group0', 'file': "hd0-file" } }

And then we insert it:

{ "execute": "x-blockdev-reopen", "arguments":
   {'driver': 'qcow2', 'node-name': 'hd0', 'file': 'throttle0'}}

So x-blockdev-reopen sees that we want to replace the current bs->file
("hd0-file") with a new one ("throttle0"). The problem here is that
throttle0 has hd0-file as its child, so when we check the permissions on
throttle0 (and its children) we get that hd0-file refuses because it's
already being used (although in in the process of being replaced) by
hd0:

"Conflicts with use by hd0 as 'file', which does not allow 'write, resize' on 
hd0-file"

And we would get a similar problem with the reverse operation (removing
the filter).

Berto