On 11/28/18 2:29 AM, Vincenzo Romano wrote:
How do you request qcow3? According to the edit history of https://
wiki.qemu.org/Features/Qcow3 , qcow3 should be many years old but the manpage
of qemu-img on my current system does not mention qcow3.
You probably mean qcow2v3. It is also spelled as 'qemu-img create -o
compat=1.1' (newer qemu-img also recognizes the spelling compat=v3), as
opposed to the older 'qemu-img create -o compat=0.10' or v2 images. But
if the virt-manager gui is specifically calling it qcow3, rather than
qcow2v3, then that's a needless confusion that we should fix in the gui.
Hi.
I relied on the "file" tool provided by Archlinux. It says "QCOW3" as
the file type by its contents.
No, it says "QCOW Image (v3)", and means qcow2v3, because file is not
distinguishing between 'qcow' and 'qcow2'. It would be worth a bug
report to the 'file' database maintainers to update their magic file to
distinguish 'qcow' (the old v1 that is no longer used anywhere) from
'qcow2' (with both 'qcow2 v2' and 'qcow2 v3' flavors).
So I have manually created another storage image file by command line with:
qemu-img create -f qcow2 ./windows2008-d1.qcow 250G
The other image file (the sparse one) has been created by virt-manager/libvirt.
So virt-manager probably requested preallocation when it created the file.
Then I have run these commands:
~ ls -l
total 2,9G
drwxrwx--- 2 vmanager vmanager 62 2018-11-27 13:14:36 .
drwxr-xr-x 7 vmanager vmanager 152 2018-11-26 12:47:06 ..
-rw-rw---- 1 libvirt-qemu kvm 6,1G 2018-11-28 09:19:22 ubuntu18.04
-rw-rw---- 1 libvirt-qemu kvm 196K 2018-11-27 13:14:36 windows2008-d1.qcow
[root@host01 /home/vmanager/storage] file *
ubuntu18.04: QEMU QCOW Image (v3), 6442450944 bytes
windows2008-d1.qcow: QEMU QCOW Image (v3), 268435456000 bytes
~ du *
3015704 ubuntu18.04
196 windows2008-d1.qcow
Both files have the same type but while "ubuntu18.04" is clearly a
sparse file, "windows2008-d1.qcow" is not.
You've got that backwards. ubuntu18.04 is sized to use 6.1G of host
space while providing 6G of data to the guest, so it is preallocated and
does not appear to be sparse (although du would let you know if the file
has holes as part of its preallocated space, such that the host size is
large but the on-disk usage is still sparse). windows2008-d1.qcow is
occupying very little host space, while advertising 250G to the guest,
so it is DEFINITELY sparse.
I doesn't make any real difference. It just puzzles me.
The difference is whether you choose to preallocate an image. There are
performance tradeoffs for doing so (a preallocated image is faster than
a sparse one when it comes to writing to previously unused portions of
the disk, but occupies more host space as a result - also, a
preallocated image tends to have less fragmentation and less chance of
hitting ENOSPC).
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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