ganeti-extstorage-interface - Specifications for ExtStorage providers


The method for supporting external shared storage in Ganeti is to have an ExtStorage provider for each external shared storage hardware type. The ExtStorage provider is a set of files (executable scripts and text files), contained inside a directory which is named after the provider. This directory must be present across all nodes of a nodegroup (Ganeti doesn’t replicate it), in order for the provider to be usable by Ganeti for this nodegroup (valid). The external shared storage hardware should also be accessible by all nodes of this nodegroup too.


There are eight required files: create, attach, detach, remove, grow, setinfo, verify (executables) and parameters.list (text file).

Common environment

All commands will get their input via environment variables. A common set of variables will be exported for all commands, and some of them might have extra ones. Note that all counts are zero-based.

Since Ganeti version 2.5, the environment will be cleaned up before being passed to scripts, therefore they will not inherit the environment in with which the ganeti node daemon was started. If you depend on any environment variables (non-Ganeti), then you will need to define or source them appropriately.

The name of the volume. This is unique for Ganeti and it uses it to refer to a specific volume inside the external storage. Its format is UUID.ext.diskX where UUID is produced by Ganeti and is unique inside the Ganeti context. X is the number of the disk count.
The volume’s size in mebibytes.
Available only to the grow script. It declares the new size of the volume after grow (in mebibytes). To find the amount of grow, the scipt should calculate the number VOL_NEW_SIZE - VOL_SIZE.
Each ExtStorage parameter (see below) will be exported in its own variable, prefixed with EXTP_, and upper-cased. For example, a fromsnap parameter will be exported as EXTP_FROMSNAP.
Available only to the setinfo script. A string containing metadata to be associated with the volume. Currently, Ganeti sets this value to originstname+X where X is the instance’s name.



The create command is used for creating a new volume inside the external storage. The VOL_NAME denotes the volume’s name, which should be unique. After creation, Ganeti will refer to this volume by this name for all other actions.

Ganeti produces this name dynamically and ensures its uniqueness inside the Ganeti context. Therefore, you should make sure not to provision manually additional volumes inside the external storage with this type of name, because this will lead to conflicts and possible loss of data.

The VOL_SIZE variable denotes the size of the new volume to be created in mebibytes.

If the script ends successfully, a new volume of size VOL_SIZE should exist inside the external storage. e.g:: a lun inside a NAS appliance.

The script returns 0 on success.


This command is used in order to make an already created volume visible to the physical node which will host the instance. This is done by mapping the already provisioned volume to a block device inside the host node.

The VOL_NAME variable denotes the volume to be mapped.

After successful attachment the script returns to its stdout a string, which is the full path of the block device to which the volume is mapped. e.g:: /dev/dummy1

When attach returns, this path should be a valid block device on the host node.

The attach script should be idempotent if the volume is already mapped. If the requested volume is already mapped, then the script should just return to its stdout the path which is already mapped to.


This command is used in order to unmap an already mapped volume from the host node. Detach undoes everything attach did. This is done by unmapping the requested volume from the block device it is mapped to.

The VOL_NAME variable denotes the volume to be unmapped.

detach doesn’t affect the volume itself. It just unmaps it from the host node. The volume continues to exist inside the external storage. It’s just not accessible by the node anymore. This script doesn’t return anything to its stdout.

The detach script should be idempotent if the volume is already unmapped. If the volume is not mapped, the script doesn’t perform any action at all.

The script returns 0 on success.


This command is used to remove an existing volume from the external storage. The volume is permanently removed from inside the external storage along with all its data.

The VOL_NAME variable denotes the volume to be removed.

The script returns 0 on success.


This command is used to grow an existing volume of the external storage.

The VOL_NAME variable denotes the volume to grow.

The VOL_SIZE variable denotes the current volume’s size (in mebibytes). The VOL_NEW_SIZE variable denotes the final size after the volume has been grown (in mebibytes).

The amount of grow can be easily calculated by the scipt and is:

grow_amount = VOL_NEW_SIZE - VOL_SIZE (in mebibytes)

Ganeti ensures that: VOL_NEW_SIZE > VOL_SIZE

If the script returns successfully, then the volume inside the external storage will have a new size of VOL_NEW_SIZE. This isn’t immediately reflected to the instance’s disk. See gnt-instance grow for more details on when the running instance becomes aware of its grown disk.

The script returns 0 on success.


This script is used to add metadata to an existing volume. It is helpful when we need to keep an external, Ganeti-independent mapping between instances and volumes; primarily for recovery reasons. This is provider specific and the author of the provider chooses whether/how to implement this. You can just exit with 0, if you do not want to implement this feature, without harming the overall functionality of the provider.

The VOL_METADATA variable contains the metadata of the volume.

Currently, Ganeti sets this value to originstname+X where X is the instance’s name.

The script returns 0 on success.


The verify script is used to verify consistency of the external parameters (ext-params) (see below). The command should take one or more arguments denoting what checks should be performed, and return a proper exit code depending on whether the validation failed or succeeded.

Currently, the script is not invoked by Ganeti, but should be present for future use and consistency with gnt-os-interface’s verify script.

The script should return 0 on success.



This file declares the parameters supported by the ExtStorage provider, one parameter per line, with name and description (space and/or tab separated). For example:

fromsnap Snapshot name to create the volume from
nas_ip The IP of the NAS appliance

The parameters can then be used during instance add as follows:

# gnt-instance add --disk=0:fromsnap="file_name",nas_ip="" ...


In the following examples we assume that you have already installed successfully two ExtStorage providers: pvdr1 and pvdr2

Add a new instance with a 10G first disk provided by pvdr1 and a 20G second disk provided by pvdr2:

# gnt-instance add -t ext --disk=0:size=10G,provider=pvdr1

Add a new instance with a 5G first disk provided by provider pvdr1 and also pass the prm1, prm2 parameters to the provider, with the corresponding values val1, val2:

# gnt-instance add -t ext

Modify an existing instance of disk type ext by adding a new 30G disk provided by provider pvdr2:

# gnt-instance modify --disk 1:add,size=30G,provider=pvdr2 <instance>

Modify an existing instance of disk type ext by adding 2 new disks, of different providers, passing one parameter for the first one:

# gnt-instance modify --disk 2:add,size=3G,provider=pvdr1,prm1=val1
                      --disk 3:add,size=5G,provider=pvdr2


Backwards compatibility

The ExtStorage Interface was introduced in Ganeti 2.7. Ganeti 2.7 and up is compatible with the ExtStorage Interface.

Common behaviour

All the scripts should display an usage message when called with a wrong number of arguments or when the first argument is -h or --help.