Datastores

OpenNebula features three different datastore types:

  • The Images Datastore, stores the images repository.
  • The System Datastore holds disk for running virtual machines, copied or cloned from the Images Datastore.
  • The Files & Kernels Datastore to store plain files.

Datastore Management

Datastores are managed with the ‘’onedatastore’’ command. In order to be operational an OpenNebula cloud needs at least one Image Datastore and one System Datastore.

Datastore Definition

A datastore definition includes specific attributes to configure its interaction with the storage system; and common attributes that define its generic behavior.

The specific attributes for System and Images Datastores depends on the storage:

Also, there are a set of common attributes that can be used in any datastore and compliments the specific attributes for each datastore type described above for each datastore type.

Attribute Description
RESTRICTED_DIRS Paths that can not be used to register images. A space separated list of paths.
SAFE_DIRS If you need to un-block a directory under one of the RESTRICTED_DIRS. A space separated list of paths.
NO_DECOMPRESS Do not try to untar or decompress the file to be registered. Useful for specialized Transfer Managers
LIMIT_TRANSFER_BW Specify the maximum transfer rate in bytes/second when downloading images from a http/https URL. Suffixes K, M or G can be used.
DATASTORE_CAPACITY_CHECK If yes, the available capacity of the datastore is checked before creating a new image
LIMIT_MB The maximum capacity allowed for the datastore in MB.
BRIDGE_LIST Space separated list of hosts that have access to the storage to add new images to the datastore.
STAGING_DIR Path in the storage bridge host to copy an Image before moving it to its final destination. Defaults to /var/tmp.
DRIVER Specific image mapping driver enforcement. If present it overrides image DRIVER set in the image attributes and VM template.
COMPATIBLE_SYS_DS Only for IMAGE_DS. Specifies which system datastores are compatible with an image datastore by ID. Ex: “0,100”

The Files & Kernels Datastore is an special datastore type to store plain files to be used as kernels, ram-disks or context files. See here to learn how to define them.

Multiple System Datastore Setup

In order to distribute efficiently the I/O of the Virtual Machines across different disks, LUNs or several storage backends, OpenNebula is able to define multiple System Datastores per cluster. Scheduling algorithms take into account disk requirements of a particular VM, so OpenNebula is able to pick the best execution host based on capacity and storage metrics.

Configuring Multiple Datastores

When more than one System Datastore is added to a cluster, all of them can be taken into account by the scheduler to place Virtual Machines into. System wide scheduling policies are defined in /etc/one/sched.conf. The storage scheduling policies are:

  • Packing. Tries to optimize storage usage by selecting the Datastore with less free space.
  • Striping. Tries to optimize I/O by distributing the Virtual Machines across Datastores.
  • Custom. Based on any of the attributes present in the Datastore template.

To activate for instance the Stripping storage policy, /etc/one/sched.conf must contain:

DEFAULT_DS_SCHED = [
   policy = 1
]

These policies may be overriden in the Virtual Machine Template, and so apply specific storage policies to specific Virtual Machines:

Attribute Description Example
SCHED_DS_REQUIREMENTS Boolean expression to select System Datastores (evaluates to true) to run a VM. SCHED_DS_REQUIREMENTS="ID=100" SCHED_DS_REQUIREMENTS="NAME=GoldenDS" SCHED_DS_REQUIREMENTS=FREE_MB > 250000
SCHED_DS_RANK Arithmetic expression to sort the suitable datastores for this VM. SCHED_DS_RANK= FREE_MB SCHED_DS_RANK=-FREE_MB

After a VM is deployed in a System Datastore, the admin can migrate it to another System Datastore. To do that, the VM must be first powered-off. The command onevm migrate accepts both a new Host and Datastore id, that must have the same TM_MAD drivers as the source Datastore.

Warning

Any Host belonging to a given cluster must be able to access any System or Image Datastore defined in that cluster.

Warning

Admins rights grant permissions to deploy a virtual machine to a certain datastore, using ‘onevm deploy’ command.

Disable a System Datastore

System Datastores can be disabled to prevent the scheduler from deploying new Virtual Machines in them. Datastores in the disabled state and monitored as usual, and the existing Virtual Machines will continue to run in them.

$ onedatastore disable system -v
DATASTORE 0: disabled

$ onedatastore show system
DATASTORE 0 INFORMATION
ID             : 0
NAME           : system
...
STATE          : DISABLED
...