[ovirt-users] Re: about the vm disk type
You mean if it's pre-allocated, it must be RAW, not Qcow2?
The documentation only states that RAW must be pre-allocated, but it does
not say that qCOW2 cannot use pre-allocation.
-----Original Message-----
From: Vojtech Juranek <vjuranek(a)redhat.com>
Sent: Wednesday, September 22, 2021 6:04 PM
To: users(a)ovirt.org
Cc: Tommy Sway <sz_cuitao(a)163.com>
Subject: Re: [ovirt-users] about the vm disk type
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.