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HOWTO09 How to use Federated Cloud Storage

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EGI Federated Cloud Storage Tutorial

This page aims to give a brief description of the storage services provided by the EGI Federated Cloud and a basic tutorial on how to use and integrate them into your application.

The guide is intended for application developers and system administrator to select the best Federated Cloud storage solution for their application needs and to understand how to integrate it into their own applications.

Storage solutions overview

If you are in need of more storage than the one provided within the VM OS image disk, you can can use the EGI Federated Cloud storage services.

There are two kind of services, the Block Storage and the Object Storage. Both of them have their own set of advantages and disadvantages

Block storage is a capability of the Federated Cloud Infrastructure-as-a-Service (IaaS). It provides additional storage blocks who can be attached to a virtual machine. A storage block is a virtual disk of a given size, which may be exposed as a virtual device in the VM. You can think of this can of devices as a USB stick that can be plugged into the VMs and can be used as a normal drive. You can format it with any file system you want and mount it in your VM. Block devices are persistent, thus they keep all the data after VM shutdown and need to be explicitly destroyed when data is not needed anymore. Block storage disks can be accessed only from within a VM, and only from VMs running at the same site where the block storage is located. Also, they can be accessed by only one VM at the same time. As part of the IaaS service, block storage is managed via OCCI (or OpenStack native interface). There is a limit on the number of block storage devices you can attach on a VM and there is a limit to the maximum size of such virtual disks. These values will depend on the particular Federated Cloud site. Moreover, the disk space is accounted for the entire block storage device, regardless how much of it is actually used.

Object storage is a standalone service of the EGI Federated Cloud, usually referred also as Federated Cloud STorage-as-a-Service (STaaS). Object storage stores data as set of individual objects, which can have different types (e.g. files, images, documents) and are organized within containers (e.g. folders). Each object/file has is own URL, which can be used to access the resource, share the file with other people, setup custom metadata and access control lists. These objects are accessed and managed via a REST API. The STaaS interface for the EGI Federated Cloud is provided via to the CDMI standard and/or OpenStack SWIFT interface. Differently from the block storage, there is virtually no limit to the amount of data you can store, only the space used is accounted, you can access the data from any location (from any VM running at any EGI site or even from other cloud providers or from your own laptop/browser), you can expose the data via external portals (using HTTP as transport protocols), you can set access control lists per container and even make the data publicly available. On the other hand, data is accessed via a API requests, thus integration with existing applications may require a change to the application logic.

A summary of the main differences between Block and Object Storage is reported in the following table.

Access Sharing Accounting Management Integration
Block Storage only from within a VM

only at the same site the VM is located

not possible for the entire block, regardless how much of it is actively used in the VM via OCCI interface (or native OpenStack for provider enabling it) POSIX access, easy with any application capable to write/read file from a local disk
Object Storage from any device connected to the internet. possible (data can be kept private or public) only for the data stored via CDMI interface (or native OpenStack for providers enabling it) files are accessed via requests to the server, requires a client to be integrated within the application

According to your application needs, you may select one technology over the other. In general, block storage is a good and simple solution for temporary data and data which you do not need to share beside the single application running on a single VM. If you need to have your data exposed within portals or shared between different steps of your processing workflow, it is usually best to use the object storage.

Block Storage

Managing Block Storage

Block storage is created and managed via requests to specific APIs. Once the storage is attached to a VM, is managed as a regular block device that can be managed from within the VM as any other block device.


The EGI Federated Cloud block storage can be managed via the OCCI interface, so you can use the rOCCI command line client as shown in the examples below. The installation guide for rOCCI client can help you to get the client ready.

To use a block storage device, you need first to create it. You can do so issuing a "create storage" OCCI command, which via rOCCI is:

occi -e <site_occi_endpoint> -n x509 -x <proxy_certificate> -X \
     -a create -r storage \


  • <site_occi_endpoint> is the OCCI endpoint of your site.
  • <proxy_certificate> is a X509 proxy certificate for authentication.
  • <storage_site_in_gb> is the size of your block storage device in GB. You will be accounted for the entire disk size, regardless how much space you are using from it. Consider also that this is the raw size of the disk. Actual available file space will depend on the file system. The minimum size is 1 (1 GB), while the maximum size depends on the site, but is usually no more than 2-5TB.
  • <storage_resource_name> is a mnemonic name for the resource. You can use this parameter internally to discriminate between disks.

That command will return the ID of the newly created storage resource (typically in the form https://<site_occi_endpoint>/storage/<some-id>). This ID will be used to identify the resource in subsequent commands.

You can list your available volumes with the list command:

occi -e <site_occi_endpoint> -n x509 -x <proxy_certificate> -X \
     -a list -r storage

And get detailed information on any of the available volumes:

occi -e <site_occi_endpoint> -n x509 -x <proxy_certificate> -X \
     -a describe -r <storage_resource_id>

After the successful creation of the storage resource, you can attach it to a VM. You can do it on an already existing VM, via the "link" command:

occi -e <site_occi_endpoint> -n x509 -x <proxy_certificate> -X \
     -a link -r <vm_id> \
     -j <storage_resource_id>

You can also attach the storage directly to a VM on creation (and this be able to use it during contextualization). Just add the --link (or the equivalent -j command to the "compute create" command:

occi -e <site_occi_endpoint> -n x509 -x <proxy_certificate> -X \
     -a create -r compute \
     [...other VM creation parameters...] \
     -j <storage_resource_id>

Please note that you can attach a storage to only one VM at the time. Any attempt to attach it to more than one VM will fail.

If a block storage is attached correctly to a VM, it will be listed as storagelink a when described, e.g:

occi -e  <site_occi_endpoint> -n x509 -x <proxy_certificate> -X \
     -a describe -r <vm_id>
    [[ ]]
    >> location: /storage/link/f9e5b73d-71ac-4abb-96c9-ce7649734ae1
    occi.core.source = /compute/2f6d70c6-fb75-4372-9917-ac688b1391ee = /storage/7cfba655-f692-406f-a659-79b0224290cc = /storage/link/f9e5b73d-71ac-4abb-96c9-ce7649734ae1
    occi.storagelink.deviceid = /dev/vdb

The occi.storagelink.deviceid shows the device on the VM where the disk is found. Block storage will be persistent, so it will not be destroyed if the VM is destroyed. It is also possible to detach it and reattach it to a different VM. Detaching is performed with this command (storage_link_id in the example above is /storage/link/f9e5b73d-71ac-4abb-96c9-ce7649734ae1):

occi -e  <site_occi_endpoint> -n x509 -x <proxy_certificate> -X \
     -a unlink -r <storage_link_id>

Once you do not need the storage anymore, you can delete it issuing a command like this:

occi -e  <site_occi_endpoint> -n x509 -x <proxy_certificate> -X \
     -a delete -r <storage_resource_id>


How to integrate the EGI Block Storage into your application

The easiest way to integrate block storage into your application is to do so at contextualization time. If you are using linux, you can use the following script as a sample for your contextualization/deployment script (NOTE: It may not work for all the OSes), which will format and mount the available storage resources into the /mnt/available_disks/ folder:

for d in xvdb xvdc xvdd vdb vdc vdd; do
  if [[ -e /dev/$d ]]; then
    if [[ ! -e /dev/${d}1 ]]; then
      echo -e "o\nn\np\n1\n\n\nw" | fdisk /dev/$d
      sleep 1
      mkfs.ext4 /dev/${d}1
    elif [[ -e /dev/${d}2 ]]; then
    mkdir -p /mnt/additional_storage/$i
    mount -t ext4 /dev/${d}1 /mnt/additional_storage/$i
    i=$(( $i + 1 ))

You can then setup your application in its deployment script to write the application files into the block storage device. Applications will not see any difference between a block storage device and a normal hardware disk, thus no major changes should be required in the application logic.

Note that some OS, like CERNVM, will automatically detect all the attached block storage and add it to the root virtual file system.

Object Storage


How to use the EGI Object Storage

Object storage does not need to be reserved a-priori, but can be used directly via the CDMI interface. Most of the CDMI operations are performed via simple HTTP calls, thus it is not strictly required to have a particular client for CDMI and generic HTTP clients like curl or wget may be used. Anyway, to access all the CDMI capabilities and manage authorization, it is recommended to use a simple CDMI client, such as bCDMI. You can install it by following the instructions here.

Access to object storage is performed via a different set of operations. Here we will try to list the most important ones and give some usage samples:

  • List the content of a folder:
[user@client]# ./bcdmi -e list /
  • Create a folder:
[[user@client]# ./bcdmi -e mkdir test
  "completionStatus": "Complete",
  "objectName": "test/",
  "capabilitiesURI": "/cdmi/AUTH_113d9a9a671944648722e890ecb94d36/cdmi_capabilities/container/",
  "parentURI": "/cdmi/AUTH_113d9a9a671944648722e890ecb94d36/",
  "objectType": "application/cdmi-container",
  "metadata": {}
  • Upload a file:
[user@client]# ./bcdmi -e put -T testfile test/test.txt
  • Download a file:
[user@client]# ./bcdmi -e get test/test.txt -o testfile
  • Delete a file:
[user@client]# ./bcdmi -e delete test/test.txt
  • Delete a folder (folder must be empty):
[user@client]# ./bcdmi -e delete test/
  • Delete a folder and all its files:
[user@client]# ./bcdmi -e delete -r test/
  • Make a folder publicly accessible:
[user@client]# ./bcdmi -e post test/ -m readacl='.r:*'
  • Make a folder private:
[user@client]# ./bcdmi -e post test/ -m readacl='.'
  • Get a public file url
[user@client]# ./bcdmi -e publicurl test/test.txt
  • Download a public file
[user@client]# curl -k

How to integrate the EGI Object Storage into your application

Integration of the block storage within your application will require a CDMI client (such as bCDMI or a set of libraries (eg. [libcdmi] to be integrated within your application. Another possibility is to integrate directly the CDMI API calls inside your application. This process may be relatively easy if you are using only the basic CDMI operations, which are standard HTTP RESTful operations. For more information, you can look at the CDMI specifications, in particular to the common operations and to the code of the bCDMI software (which uses simple cURL calls to perform the CDMI operations).

Within CDMI, it is possible to set a file or a container as public (see paragraph above). If so, read operations can be performed via normal HTTP calls without authentication. This means you can access the data with any web browser and seamlessly integrate it within any web portal served by HTTP.

Other CDMI operations (like write operations), requires authentication. For the FedCloud, this is performed via a access token generated from the Keystone service of the site using the user X509-VOMS proxy certificate. It is possible to use the bCDMI client to perform the authentication process via the auth command (see bCDMI usage guide for more information) and then use the generated token in your application with normal HTTP calls.