Raw Device Mapping (RDM) Datastore

The RDM Datastore is an Image Datastore that enables raw access to block devices on Nodes. This Datastore enables fast VM deployments due to a non-existent transfer operation from the Image Datastore to the System Datastore.

Warning

The Datastore should only be usable by the administrators. Letting users create images in this Datastore is a huge security risk! For example, users could register an image /dev/sda and read the Host filesystem.

Datastore Layout

The RDM Datastore is used to register already existing block devices on the Nodes. The devices should be already set up and available, and VMs using these devices must be enforced to run on the Nodes ready for them. Additional Virtual Machine files, like deployment files or volatile disks, are created as regular files.

Front-end Setup

No additional setup is required. Make sure /etc/one/oned.conf has the following configuration for the RDM Datastore:

TM_MAD_CONF = [
    NAME = "dev", LN_TARGET = "NONE", CLONE_TARGET = "NONE", SHARED = "YES",
    TM_MAD_SYSTEM = "ssh,shared",
    LN_TARGET_SSH = "SYSTEM", LN_TARGET_SHARED = "NONE",
    DISK_TYPE_SSH = "FILE", DISK_TYPE_SHARED = "FILE",
    CLONE_TARGET_SSH = "SYSTEM", CLONE_TARGET_SHARED =  "SELF",
    DRIVER = "raw"
]

Node Setup

The devices you want to attach to a VM should be accessible by the hypervisor. KVM usually runs under the identity of oneadmin user; make sure this user is in a group with access to the disk (like disk) and has read write permissions for the group.

OpenNebula Configuration

Once the Node storage setup is ready, the OpenNebula configuration comprises two steps:

  • Create System Datastore
  • Create Image Datastore

Create System Datastore

The RDM Datastore can work with the following System Datastores:

Note that the System Datastore is only used for volatile disks and context devices.

Create Image Datastore

To create an Image Datastore, you need to set following (template) parameters:

Attribute Description
NAME Name of Datastore
TYPE IMAGE_DS
DS_MAD dev
TM_MAD dev
DISK_TYPE BLOCK

An example template of Datastore:

$ cat rdm.conf
NAME      = rdm_datastore
TYPE      = "IMAGE_DS"
DS_MAD    = "dev"
TM_MAD    = "dev"
DISK_TYPE = "BLOCK"

$ onedatastore create rdm.conf
ID: 101

Datastore Usage

New images can be added just like any other image by specifying the path. As an example, here is an image template to add a Node disk /dev/sdb:

$ cat image.tmpl
NAME=scsi_device
PATH=/dev/sdb
PERSISTENT=YES

$ oneimage create image.tmpl -d 101

If you are using the CLI, pass the absolute path to the device with parameter --source:

oneimage create -d 101 --name nbd --source /dev/sdc --driver raw --prefix vd --persistent --type OS --size 0MB

Note

As this Datastore is just a container for existing devices, the size of images in OpenNebula doesn’t reflect the real block device size. All devices registered will render a size of 0 and the overall devices Datastore will show up with 1MB of available space