[ovirt-users] Clone, template, pools : how does it uses disk space ?

[Fred Rolland]

Hi,

NFS will be sparse. Preallocate is relevant for block storage.

There is a fix to change the UI to display the format (RAW/QCOW2) of the
disk instead of the type (Sparse/Preallocated)  [1].
When creating a VM from a template, If you choose thin provisioning the
disk format will be alway QCOW2, keeping the disk from the template as is
read-only and creating a disk only for the diffs.
If you choose Clone, then the whole disk will be copied and you will have a
new copy of template disk in whatever format you want (Raw or QCOW2).

>What happens if you clone a source VM which is using "thin" ?
The VM cloned from a thin templated base VM will be created with a full
copy of the disk (as in "Clone" when creating from template)

>Interesting. Is there a way to "merge" the changes ? (I mean to change it
from being "thin provision" after its creation and make it an independent
VM)
No, a VM created as thin will remain like that. You can clone a new VM from
it and them you will have an "independent VM"

>When you create a template for the first time, it seems you can't choose
between clone and thin, which one is used ?
When you create a template you make a read-only copy of the VM disks
(Clone) . You can choose the format of the disks (RAW/QCOW2)

[1] https://gerrit.ovirt.org/#/c/57190/

On Mon, May 23, 2016 at 3:50 PM, Alexis HAUSER <
alexis.hauser at telecom-bretagne.eu> wrote:

> >Regarding your examples, I cannot say exactly because of lack of some
> >details. What storage type are you using? How do you measure the space
> used
> >on the physical disk?
>
>
> simply df -h on the PC sharing the NFS storage.
>
>
> >> For example, when making a VM from template, using pre-allocated disk
> >> option, for a 50GB Virtual disk, it only uses 3GB on the physical disk.
>
> > 3GB is the VM's disk? What about the disk of the template?
>
>
> 3GB is the difference using df -h betweem before making the template, and
> after making it and running the VM.
>
>
> >Generally, 50GB pre-allocated disk will take 50GB of physical space. A
> 50GB
> >sparse disk will take as many 1GB chunks as needed to store all the
> >information that was written to it, maximum 50GB.
>
>
> so "pre-allocated" doesn't use pre-allocation but sparse instead ? I don't
> really get it, sorry.
>
>
> >When you create a VM by cloning another VM or create a VM from a template
> >in "clone" mode, a copy of the source disk will be created. The new disk
> >will take as much space as the source disk did.
>
>
> What happens if you clone a source VM which is using "thin" ?
>
>
> >When you create a VM from a template in "thin provision" mode or creating
> a
> >VM in a pool, the new disk will be initially only a reference to the
> source
> >disk. Reading from it will read the source disk. Writing to it will write
> >to the new disk, not touching the source. Thus, all disk fragments that
> >were overwritten after disk creation will be physically stored in the new
> >disk and read from it, those that were not overwritten, will be read from
> >the source disk.
>
>
> Interesting. Is there a way to "merge" the changes ? (I mean to change it
> from being "thin provision" after its creation and make it an independent
> VM)
> When you create a template for the first time, it seems you can't choose
> between clone and thin, which one is used ?
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