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

[Alberto Garcia]

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.

Berto