For example :

And I check the file on the storage:

[root@olvms1 51dcbfae-1100-4e43-9e0a-bb8c578623d7]# cat 9e4dc022-c450-4f85-89f5-233fa41c07d0.meta

CAP=10737418240

CTIME=1632305740

DESCRIPTION={"DiskAlias":"test09222_Disk1","DiskDescription":""}

DISKTYPE=DATA

DOMAIN=f77091d9-aabc-42db-87b1-b8299765482e

FORMAT=RAW

GEN=0

IMAGE=51dcbfae-1100-4e43-9e0a-bb8c578623d7

LEGALITY=LEGAL

PUUID=00000000-0000-0000-0000-000000000000

TYPE=SPARSE

VOLTYPE=LEAF

EOF

[root@olvms1 51dcbfae-1100-4e43-9e0a-bb8c578623d7]#

[root@olvms1 51dcbfae-1100-4e43-9e0a-bb8c578623d7]# ll

total 1025

-rw-rw----. 1 vdsm kvm 10737418240 Sep 22 18:15 9e4dc022-c450-4f85-89f5-233fa41c07d0

-rw-rw----. 1 vdsm kvm 1048576 Sep 22 18:15 9e4dc022-c450-4f85-89f5-233fa41c07d0.lease

-rw-r--r--. 1 vdsm kvm 303 Sep 22 18:15 9e4dc022-c450-4f85-89f5-233fa41c07d0.meta

[root@olvms1 51dcbfae-1100-4e43-9e0a-bb8c578623d7]#

[root@olvms1 51dcbfae-1100-4e43-9e0a-bb8c578623d7]# du -h ./9e4dc022-c450-4f85-89f5-233fa41c07d0

0 ./9e4dc022-c450-4f85-89f5-233fa41c07d0

[root@olvms1 51dcbfae-1100-4e43-9e0a-bb8c578623d7]#

-----Original Message-----
From: users-bounces@ovirt.org <users-bounces@ovirt.org> On Behalf Of Tommy Sway
Sent: Wednesday, September 22, 2021 6:07 PM
To: 'Vojtech Juranek' <vjuranek@redhat.com>; users@ovirt.org
Subject: [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@redhat.com>

Sent: Wednesday, September 22, 2021 6:04 PM

To: users@ovirt.org

Cc: Tommy Sway <sz_cuitao@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.

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