[pve-devel] [RFC storage] work-around #6543: do not use preallocation for qcow2 on top of LVM

[DERUMIER, Alexandre]

>>Currently, the
>>option does not seem to make a difference with 'qemu-img measure', so
>>that needs to be further investigated. 

it seem to be normal that preallocation option don't make a difference
here. qemu-img measure always compute size of all metadatas.  (only
cluster_size && l2_extended change the result)


I don't understand why disabling preallocation could help here (until
it's bugged), because the same amount of metadatas need to be allocated
later on the fly  with disabled preallocation.