[one-users] qcow2 images in datastores and as volatile disks

[Ruben S. Montero]

>  1.: I have to set FORMAT = "qcow2" so qemu-img create is used to create
> the image file. But this way I can not specify a filesystem to be created,
> right? I have to manually format the disk after vm booted (of curse I can
> use contextualisation to do this part). Is there a way to tell opennebula
> to create qcow2 img and then format it with ext3?
>

Exactly, you need to mount the disk and format it within the VM, once
booted. We thought about adding an extra TYPE something like qcow2+ext3 but
we feel that would be more confusing...


>
> 2.: The main reason why I want to use qcow2 is thin provisioning in the
> meaning of empty disk does not use $max_size at our storage. For testing I
> created a disks like:
>
> DISK  = [ TYPE = "fs",
>           SIZE = 40000,
>           FORMAT = "ext3"]
>
> DISK  = [ TYPE = "fs",
>           SIZE = 40000,
>           FORMAT = "raw"]
>
> I saw that even this disks only took ~750Mb on our NFS server. Can someone
> explain why, I expected that these raw (or raw with ext3 filesystem on it)
> will take the whole 40k Mb on our storage.
>

Maybe that's because you are using a file system with support for sparse
files.


>
> 3.: The transfer manager ist attached to a datastore and not to a image.
> So I can not put qcow2 and raw images into the same datastore. Is there a
> reason for that?
>

Actually you can. You'll have a datastore of type fs (DS_MAD = fs) and
using a transfer manager drivers qcow (TM_MAD=qcow2). You can create "raw"
files in the datastore. The TM driver will use qemu-img commands, but AFAIK
that also handle raw images.


> Why can't we attach the tm to a image instead of datastore?
>

TM is to specify how the images of a datastore are distributed to the
hosts. This is related to the storage configuration (shared by all the
images in a given datastore) and not to the image itself


> but I can put qcow2 images into shared tm datastore if the vm template
> overrides this by setting driver=qcow2, right?
>

Yes driver is an attribute of the image (the image format), you need to add
the driver=qcow2 for the qcow2 images.

Summary.

1.- Define a fs datastore [1] with the qcow2 transfer driver
2.- Create images in it, either register exisiting qcow2 files, or creating
datablocks of type qcow2 (to be formatted on the vm) or any other type.
Qcow2 images would need DRIVER=qcow2
3.- Use those files in your VMs with DISK = [...]

NOTE: Volatile disks as the one in your email are created on the fly in the
system datastore. [2]


Up to now I only thought about different datastores are different storage
> locations and different redundancy zones/slas/performance classes. Any
> clearification is appreciated
>

Yes, basically datastores are abstractions to manage different storage
architectures and configurations. Balancing among different servers, slas
and the like are goodies that comes that abstraction.


[1]
http://opennebula.org/documentation:rel3.8:fs_ds#using_the_qcow2_transfer_driver
[2] http://opennebula.org/documentation:rel3.8:system_ds

Cheers

Ruben

>
> Thanks for your help,
> Nils Schröder
>
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-- 
Ruben S. Montero, PhD
Project co-Lead and Chief Architect
OpenNebula - The Open Source Solution for Data Center Virtualization
www.OpenNebula.org | rsmontero at opennebula.org | @OpenNebula
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