On Thu 16 Apr 2015 02:27:39 PM CEST, Eric Blake wrote:
>>>> + orig_bs_flags = bdrv_get_flags(bs);
>>>> + if (!(orig_bs_flags & BDRV_O_RDWR)) {
>>>
>>> I feel like we don't want to do this if we're not streaming to an
>>> intermediate layer but to the top layer (because that means there is
>>> some reason for the BDS to be read-only beyond it just being a
>>> backing BDS).
>>
>> I didn't think about this... that I can fix easily, but I wonder
>> what's the scenario where the top layer is read-only.
>
> Suppose I tell qemu to open the chain 'base <- mid <- top' read-only,
> and then later decide to stream base into mid or top. It should be
> fine from the guest's perspective to keep the guest in read-only mode,
> while still using streaming to reduce the chain.
Ok, I'll add the check for the top image then.
>> I actually tried with scenarios such as A>B>C>D>E, streaming from B
>> to E and from A to C simultaneously, and it seems to work (at least I
>> didn't see any obvious problems), but I don't think we want to
>> support that because a) I don't see the use case and b) we're likely
>> opening a can of worms.
>
> Streaming doesn't corrupt any part of the chain, so I think you are
> safe doing overlapping streams like that. However, I also agree that
> it is not worth supporting, and that we are safer not allowing
> overlapping streams.
I'm thinking about cases where streaming from B to E finishes (and all
the intermediate nodes are removed from the chain) while A to C is still
ongoing. At the very least it doesn't seem to make sense to allow that
kind of things.
Berto
