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

[Shmuel Melamud]

Hi!

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?

> 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?

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.

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.

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.

Also note that pool VMs are stateless, so the information that was written
to their disks when VM was used by user is erased after the VM is returned
back to the pool.

Shmuel

On Mon, May 23, 2016 at 12:50 PM, Alexis HAUSER <
alexis.hauser at telecom-bretagne.eu> wrote:

> Hi,
>
>
> I would like to know what happens to storage when using the different
> method of cloning or generating VMs using templates / pools.
> I'd like to know also in what case VM and virtual disks are totally
> independent and in what case they are not.
> Sadly the RHEV documentation doesn't really provide these informations and
> I don't find any explicit informations about it.
>
> 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.
> Another example, when making a pool of 10 VMs, based on a VM with a 50 GB
> virtual disk, only 2GB more space is used on the physical disk.
> What is exactly done when this happens ?
>
> Here are the case I would like to have informations about (physical
> storage, and independence of VMs) :
> - using simple "clone function"
> - making VM from template with "clone" mode
> - making VM from template with "thin" mode
> - making VM in pools
>
> Is there modes calculating only the difference from the original VM, and
> other modes copying totally the informations from the virtual disk from the
> original VM ?
>
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>
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