When I create the VM's image disk, I am not asked to select the following type of disk.

What is the default value ?

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

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.

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]# [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]# [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]# [root@olvms1 51dcbfae-1100-4e43-9e0a-bb8c578623d7]# -----Original Message----- From: users-bounces(a)ovirt.org <users-bounces(a)ovirt.org&gt; On Behalf Of Tommy Sway Sent: Wednesday, September 22, 2021 6:07 PM To: 'Vojtech Juranek' <vjuranek(a)redhat.com>; users(a)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(a)redhat.com&gt; Sent: Wednesday, September 22, 2021 6:04 PM To: users(a)ovirt.org Cc: Tommy Sway <sz_cuitao(a)163.com&gt; 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. _______________________________________________ Users mailing list -- users(a)ovirt.org To unsubscribe send an email to users-leave(a)ovirt.org Privacy Statement: https://www.ovirt.org/privacy-policy.html oVirt Code of Conduct: https://www.ovirt.org/community/about/community-guidelines/ List Archives: https://lists.ovirt.org/archives/list/users@ovirt.org/message/JGJX4VUOYVB... _______________________________________________ Users mailing list -- users(a)ovirt.org To unsubscribe send an email to users-leave(a)ovirt.org Privacy Statement: https://www.ovirt.org/privacy-policy.html oVirt Code of Conduct: https://www.ovirt.org/community/about/community-guidelines/ List Archives: https://lists.ovirt.org/archives/list/users@ovirt.org/message/JMXUYJXYWNH...

I also have block storage on my environment. How do I observe the type of a vm image (LV)?

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;

If Thin mode, is QCOW2;

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.

If the vm is cloned from a Thin template, I haven't had time to test this.

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.