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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