Re: [Qemu-devel] [Qemu-block] [PATCH v5 3/4] qmp: add monitor command to add/remove a child

[Kevin Wolf <kwolf@redhat.com>]

Am 08.10.2015 um 10:29 hat Alberto Garcia geschrieben:
> On Thu 08 Oct 2015 08:15:25 AM CEST, Markus Armbruster wrote:
> >> For the second point, you should also consider how useful this
> >> feature is to management tools. Just being able to remove and attach
> >> children from a quorum node seems very useful on its own. I don't see
> >> why we should wait for having support for other block drivers; also,
> >> for most block drivers there is no meaningful way of adding or
> >> removing children as nicely as that is possible for quorum.
> >
> > Okay, this is an argument I might be able to buy.
> 
> Note that if we want to make this interface stable there's one use case
> missing: there's currently no way to change the vote threshold.
> 
> This is maybe not so important for the COLO use case, but for the
> general case of adding and removing children from a quorum node having
> the possibility to change the threshold makes a lot of sense.
> 
> That would probably require a its own API ('quorum-set-threshold' or
> something like that) so I don't think it has an effect on these
> child-add and child-del commands, but I wanted to mention it here anyway
> in case someone sees something that I'm overlooking.

The right way to change the voting threshold is bdrv_reopen(). Adding
support to quorum for changing the threshold this way should be trivial.
The hard part is how to expose it in QMP, which is why I've only added
it to qemu-io so far (which happens to be accessible from HMP).

If you need this functionality, let's start a new email thread and
discuss the right QMP interface for blockdev-reopen (or blockdev-
set-options or whatever you would call it).

Kevin