Fam Zheng <f...@redhat.com> writes:
> 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?
Thanks! I figure your placeholder node does immunize the splicing in of
the throttle-filter against the removal of mirror-filter.
Given a specific dynamic reconfiguration, where do we have to put
placeholder nodes to immunize it against arbitrary other dynamic
changes?
