[virt-tools-list] Storage image files and permissions

[Eric Blake]

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 at 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