[libvirt] [RFC v2] external (pull) backup API

[Vladimir Sementsov-Ogievskiy]

13.04.2018 21:02, John Snow wrote:
>
> On 04/13/2018 08:01 AM, Vladimir Sementsov-Ogievskiy wrote:
>>>> 1. It looks unsafe to use nbd server + backup(sync=none) on same node,
>>>> synchronization is needed, like in block/replication, which uses
>>>> backup_wait_for_overlapping_requests, backup_cow_request_begin,
>>>> backup_cow_request_end. We have a filter driver for this thing, not yet
>>>> in upstream.
>>> Is it the case that blockdev-backup sync=none can race with read
>>> requests on the NBD server?
>>>
>>> i.e. we can get temporarily inconsistent data before the COW completes?
>>> Can you elaborate?
>> I'm not sure but looks possible:
>>
>> 1. start NBD read, find that there is a hole in temporary image, decide
>> to read from active image (or even start read) and yield
>> 2. guest writes to the same are (COW happens, but it doesn't help)
>> 3. reduce point (1.), read invalid (already updated by 2.) data
>>
>> And similar place in block/replication, which uses backup(sync=none) too
>> is protected from such situation.
> I'll have to look into this one -- were you seeing problems in practice
> before you implemented your proprietary filter node?
>
> --js

I didn't see problems, I just noted, that it is done in 
block/replication and looked through corresponding commit messages.

-- 
Best regards,
Vladimir