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

Alexis HAUSER alexis.hauser at telecom-bretagne.eu
Mon May 23 12:50:08 UTC 2016

>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 ?

More information about the Users mailing list