VT-CloudCaps:BestPractices

This page details the dos and don'ts of image creation when it comes to leveraging various cloud capabilities and achieving multi-platform compatibility.

Platform-specific practices

OpenNebula

If you want to leverage some platform-specific features provided by OpenNebula and use workflows endorsed by its community, you should follow a few recommendations regarding image creation. This is especially important for users coming to OpenNebula from other cloud platforms, such as OpenStack. In order to understand these recommendations, you first have to understand how OpenNebula works with block storage, images and virtual machines in general.

Images

When you upload a file containing the file system of a virtual machine (image of a single partition or the whole disk, it doesn't matter at this point) to OpenNebula and register it as an IMAGE, it becomes attachable but not resizeable. This means you will be able to create a virtual machine using this IMAGE but you won't be able to change its size during instantiation or expand its file system to fill a bigger IMAGE.

The only way to add more block storage to a virtual machine is to create a special empty IMAGE using type DATABLOCK and attach it as a new block device. This means that storage space can be added only to a new mount point inside the virtual machine.

Examples

• Example #1

I have a raw disk image containing an MBR and a single partition
with the root file system of an arbitrary Linux distribution.

This image has the following attributes:
 -- 4 GB of space in total
 -- 2 GB taken by the OS installation
 -- no fs encryption, no LVM, no swap
 -- credentials for root access are already included
 -- everything is stored in a single file my_cloud_vm.img

Once I upload and register this file as an IMAGE in OpenNebula, it
becomes available as IMAGE_ID = 1 and I will use it to instantiate
a virtual machine.

After a successful login into the running virtual machine, I realize
that there is not enough space to install the software or copy the
data I need to run my computing job.

The only way to solve this problem /without expanding, uploading
and registering a new (and bigger) IMAGE/ is to attach a DATABLOCK
and mount it inside the virtual machine as a new mount point.

There is no way to expand the existing root file system or IMAGE
itself once it is registered in OpenNebula.

• Example #2