Re: [Qemu-devel] [PATCH 2/5] block/qcow2-bitmap: Allow bitmap flushing

[John Snow]

On 3/6/19 11:12 AM, Vladimir Sementsov-Ogievskiy wrote:
> 06.03.2019 18:59, John Snow wrote:
>>
>>
>> On 3/6/19 7:58 AM, Eric Blake wrote:
>>> On 3/5/19 5:43 PM, John Snow wrote:
>>>> Usually, we only write out bitmaps when we're about to close out the file,
>>>> so we always remove the bitmaps after to make it easier to determine the
>>>> source of truth for migration purposes.
>>>>
>>>> However, there may be times we want to flush bitmap data more aggressively,
>>>> like after a truncate event where we need to make the metadata consistent
>>>> again.
>>>
>>> Or, as I've mentioned in other threads, after every
>>> bitmap-add/bitmap-delete operation, so that 'qemu-img info -U' can see
>>> which persistent bitmaps exist. But that's one step further than this
>>> series, so I won't insist on it today.
>>>
>>
>> Yes, I was keeping that in mind as I wrote this. I think it extends to
>> those cases trivially, so long as Vladimir is OK with what I'm trying to do.
>>
> 
> I still don't see the point of flushing bitmaps, the only thing is optimizing
> qemu-img info --force-share, which should never used except for debugging.
> 
> So, at least, if we'll have it, I'd prefer bitmap flushing to be optional,
> and other code should not rely on it.
> 

I don't have plans to add more uses of it personally, but I do think
that resize represents a genuine case of wanting the feature.