According to what you said, qemU-img info tool was used to query the result. On Block storage, theses are results: if preallocated, the storage format is RAW;
correct
If Thin mode, is QCOW2;
correct
If the VM is cloned from a template in preallocated mode, the disk consists of two files: the bottom back-end file is RAW, and the front end file is QCOW2.
correct, second one is snapshot (not to modify template image) and snaphosts are always qcow2
If the vm is cloned from a Thin template, I haven't had time to test this.
both disks should be qcow2
According to the above classification, the performance of QCOW2 is worse than that of RAW format. Therefore, using preallocation mode for Block storage can not only remove interference at the file system level, but also achieve performance advantages in file types, thus achieving the best I/O effect.
The last sentence was the result I really wanted, as I was running IO heavy systems on OLVM.
if you want to use only raw file format, you should use preallocated disks on block storage (on file storage doesn't matter) and avoid snapshots, as snapshots are always qcow2 format