Re: [Qemu-block] [Qemu-devel] Meeting notes on -blockdev, dynamic backend reconfiguration

[Markus Armbruster]

Fam Zheng <address@hidden> writes:

> On Thu, 12/08 13:46, Markus Armbruster wrote:
>> Fam Zheng <address@hidden> 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?