On Wednesday, 22 September 2021 09:55:26 CEST Tommy Sway wrote:
> When I create the VM's image disk, I am not asked to select the following
> type of disk.

Actually you are, it's "Allocation Policy" drop down menu.
Thin provisioned == qcow format
Preallocated == raw

> 
> 
> What is the default value ?

Thin provisioned, i.e. qcow.

> 
> 
> Thanks.
> 
> 
> 
> 
> 
> QCOW2 Formatted Virtual Machine Storage
> 
> QCOW2 is a storage format for virtual disks. QCOW stands for QEMU
> copy-on-write. The QCOW2 format decouples the physical storage layer from
> the virtual layer by adding a mapping between logical and physical blocks.
> Each logical block is mapped to its physical offset, which enables storage
> over-commitment and virtual machine snapshots, where each QCOW volume only
> represents changes made to an underlying virtual disk.
> 
> The initial mapping points all logical blocks to the offsets in the backing
> file or volume. When a virtual machine writes data to a QCOW2 volume after a
> snapshot, the relevant block is read from the backing volume, modified with
> the new information and written into a new snapshot QCOW2 volume. Then the
> map is updated to point to the new place.
> 
> Raw
> 
> The raw storage format has a performance advantage over QCOW2 in that no
> formatting is applied to virtual disks stored in the raw format. Virtual
> machine data operations on virtual disks stored in raw format require no
> additional work from hosts. When a virtual machine writes data to a given
> offset in its virtual disk, the I/O is written to the same offset on the
> backing file or logical volume.
> 
> Raw format requires that the entire space of the defined image be
> preallocated unless using externally managed thin provisioned LUNs from a
> storage array.

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