Re: [Qemu-devel] [RFC] Re-evaluating subcluster allocation for qcow2 images

[Denis Lunev]

On 6/28/19 6:02 PM, Alberto Garcia wrote:
> On Fri 28 Jun 2019 04:57:08 PM CEST, Kevin Wolf wrote:
>> Am 28.06.2019 um 16:43 hat Alberto Garcia geschrieben:
>>> On Thu 27 Jun 2019 06:05:55 PM CEST, Denis Lunev wrote:
>>>> Please note, I am not talking now about your case with COW. Here the
>>>> allocation is performed on the sub-cluster basis, i.e. the abscence of
>>>> the sub-cluster in the image means hole on that offset. This is
>>>> important difference.
>>> I mentioned the possibility that if you have a case like 2MB / 64KB
>>> and you write to an empty cluster then you could allocate the
>>> necessary subclusters, and additionally fallocate() the space of the
>>> whole cluster (2MB) in order to try to keep it contiguous.
>>>
>>> With this we would lose the space saving advantage of having
>>> subclusters. But perhaps that would work for smaller cluster sizes
>>> (it would mitigate the fragmentation problem).
>> There seem to be use cases for both ways. So does this need to be an
>> option?
> Probably a runtime option, or a heuristic that decides what to do
> depending on the cluster size.
no, I think that this should be on-disk option as this affects
allocation strategy.

Den