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

[Alberto Garcia <berto@igalia.com>]

On Fri 28 Jun 2019 05:03:13 PM CEST, Denis Lunev wrote:
> On 6/28/19 6:02 PM, Alberto Garcia 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.

But why does it need to be stored on-disk? It should be theoretically
possible to switch between on strategy and the other at runtime (not
that it would make sense though).

Berto