Re: [Qemu-devel] [PATCH] block/snapshot: dirty all dirty bitmaps on snapshot-switch

[John Snow <jsnow@redhat.com>]

On 11/17/2017 10:01 AM, Max Reitz wrote:
> On 2017-11-17 13:30, Kevin Wolf wrote:
>> Am 23.10.2017 um 11:29 hat Vladimir Sementsov-Ogievskiy geschrieben:
>>> Snapshot-switch actually changes active state of disk so it should
>>> reflect on dirty bitmaps. Otherwise next incremental backup using
>>> these bitmaps will be invalid.
>>>
>>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>>
>> We discussed this quite a while ago, and I'm still not convinced that
>> this approach makes sense.
> 
> I think it at least makes more sense than not handling this case at all.
> 
>> Can you give just one example of a use case where dirtying the whole
>> bitmap while loading a snapshot is the desired behaviour?
>>
>> I think the most useful behaviour would be something where the bitmaps
>> themselves are snapshotted, too.
> 
> Agreed.
> 
>>                                  But for the time being, the easiest and
>> safest solution might just be to error out in any snapshot operations
>> if any bitmaps are in use.
> 
> Sounds OK, too.  I personally don't have an opinion either way.
> 
> But in any case, what we did before this patch was definitely wrong so I
> consider it an improvement.
> 

This is how I feel about it too. Erroring out entirely is an option, but
code-wise just dirtying everything is at least verifiably not-wrong and
pretty simple to implement.

It's an improvement... Don't do it, but at least you won't get something
wrong after, just something heinously unoptimal.

> Max
>