On Thu, 12/08 13:46, Markus Armbruster wrote:
> Fam Zheng <f...@redhat.com> writes:
>
> > On Wed, 12/07 10:48, Kevin Wolf wrote:
> >> > If so I think there is no race to worry about, mirror-filter should go
> >> > away only after a QMP command.
> >>
> >> Currently, a mirror job goes away whenever it is done. This is not
> >> directly tied to a QMP command.
> >
> > Ah right, block-job-complete is only "start to complete" and the job goes
> > away
> > at some later point. I thought this is "the" QMP command but it is not.
> >
> >>
> >> Of course, in the new job API we want an explicit job-delete, so in
> >> that case it wouldn't happen, but we need to keep the old case for
> >> compatibility.
> >
> > Another possibility is to add a placeholder node in the right location first
> > then fill in the actual throttling node once created. QMP owns the
> > placeholder
> > node so it won't suddenly vanish when mirror job goes away.
>
> I'm not sure I understand this idea. Could you explain it in a bit more
> detail, perhaps even with a bit of ASCII art?
OK, I'll base on your version:
== Basic dynamic reconfiguration operation ==
The two primitives are "insert placeholder" and bdrv_replace_child;
A placeholder BDS is a dummy "pass through everything" filter. Inserting
placeholder is something like:
void bdrv_insert_placeholder(BdrvChild *child) {
BlockDriverState *ph = bdrv_new_placeholder();
ph->file->bs = child->bs;
child->bs = ph;
}
Consider:
BB
|
mirror-filter
|
BDS
Add a throttle filter under BB while the mirror job is running. First step,
create the dummy _placeholder_ node and throttle-filter node (the existence of
placeholder node can be an implementation detail hidden from user):
placeholder = bdrv_insert_placeholder(BB->root);
throttle_filter = bdrv_create_throttle_filter(placeholder);
we get:
BB throttle-filter
| /
placeholder
|
mirror-filter
|
BDS
Second step, move throttle filter in:
bdrv_replace_child(BB->root, throttle_filter);
bdrv_replace_child(throttle_filter->file,
throttle_filter->file->bs->file->bs);
bdrv_unref(placeholder);
we get:
BB
|
throttle-filter
|
mirror-filter
|
BDS
Does this make us safe from racing with mirror-filter disappearing?
Fam
