The iSCSI Datastore 3.4

Overview

The iSCSI datastore driver provides OpenNebula with the possibility of using block-devices for VM images instead of the default file form. The use of the iSCSI datastores presents several benefits, especially regarding performance.

It is developed to work with tgtd, the Linux SCSI target framework. However this driver is written in a way it can be easily hacked into working with specific SAN systems or other iSCSI target softwares.

There are some limitations that you have to consider, though:

  • No support for non-persistent images. This driver only provides persistent images. Every image created in a datastore that uses iSCSI drivers should be made persistent.
  • No iSCSI ACLs. This driver doesn't implement ACLs or zoning for the targets defined in the server. This means that every host will be able to see through discovery all the targets exported by the iSCSI target server, as long as it can make a connection to the host on the iscsi port.

:!: Note that it is possible to use images that belong to different datastores in the same VM. Non-persistent images from other datastore types can be used in a VM along with the persistent iSCSI images.

Requirements

OpenNebula Front-end

  • Password-less ssh access to the iSCSI target as the oneadmin user

The iSCSI Target

  • Password-less sudo permission for: tgtadm, lvcreate, lvremove and dd.
  • LVM2
  • Linux SCSI target framework (tgt).

OpenNebula Hosts

  • oneadmin user should belong to the disk group for the KVM hypervisor. This is needed when dynamic_ownership is not set.

Configuration

Configuring the System Datastore

To use iSCSI drivers, you must configure the system datastore as shared. This sytem datastore will hold only the symbolic links to the block devices, so it will not take much space. See more details on the System Datastore Guide

It will also be used to hold context images and Disks created on the fly, they will be created as regular files.

Configuring iSCSI Datastores

The first step to create a iSCSI datastore is to set up a template file for it. In the following table you can see the supported configuration attributes. The datastore type is set by its drivers, in this case be sure to add DS_MAD=iscsi and TM_MAD=iscsi for the transfer mechanism, see below.

Attribute Description
NAME The name of the datastore
DS_MAD The DS type, use iscsi for the iSCSI datastore
TM_MAD Transfer drivers for the datastore, use iscsi, see below
DISK_TYPE Type for the VM disks using images from this datastore. Supported values are: block, file
HOST The tgt server host. Defaults to localhost
VG_NAME The LVM volume group name. Defaults to vg-one
BASE_IQN The base IQN for iSCSI target. Defaults to iqn.2012-02.org.opennebula
BASE_TID Starting target ID. Defaults to 1
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.

:!: This will prevent users registering important files as VM images and accessing them through their VMs. OpenNebula will automatically add its configuration directories: /var/lib/one, /etc/one and oneadmin's home. If users try to register an image from a restricted directory, they will get the following error message: “Not allowed to copy image file”.

For example, the following examples illustrates the creation of a iSCSI datastore using a configuration file. In this case we will use the host iscsitarget as our iSCSI target server.

> cat ds.conf
NAME = production
DS_MAD = iscsi
TM_MAD = iscsi
VG_NAME = vg-one
HOST = iscsitarget

> onedatastore create ds.conf
ID: 100

> onedatastore list
  ID NAME            CLUSTER  IMAGES TYPE   TM    
   0 system          none     0      fs     shared
   1 default         none     3      fs     shared
 100 production      none     0      iscsi  shared

You can check more details of the datastore by issuing the onedatastore show command.

:!: Note that datastores are not associated to any cluster by default, and they are supposed to be accessible by every single host. If you need to configure datastores for just a subset of the hosts take a look to the Cluster guide.

Using the iSCSI transfer driver

The iSCSI transfer driver will issue a iSCSI discover command in the target server with iscsiadm. Once the block device is available in the host, the driver will login, mount it and link it to disk.i.

Host Configuration

The hosts must have Open-iSCSI installed, which provides iscsiadm.

In order for iscsiadm to work, it needs to be able to make a connection on the default iSCSI port (3260) with the iSCSI target server. Firewalls should be adjusted to allow this connection to take place.

The oneadmin user must have sudo permissions to execute iscsiadm. Additionally, the oneadmin user should to the disk group.

Tuning & Extending

System administrators and integrators are encouraged to modify these drivers in order to integrate them with their iSCSI SAN/NAS solution. To do so, the following is a list of files that may be adapted:

Under /var/lib/one/remotes/:

  • datastore/iscsi/iscsi.conf: Default values for iSCSI parameters
    • HOST: Default iSCSI target host
    • BASE_IQN: Default IQN path
    • VG_NAME: Default volume group
    • BASE_TID: Starting TID for iSCSI targets
    • NO_ISCSI: Lists of hosts (separated by spaces) for which no iscsiadm login or logout.
  • scripts_common.sh: includes all the iSCSI methods:
    • tgtadm_target_new(target_id, iqn): Registers a new target for a given IQN
    • tgtadm_target_bind_all(target_id): Allows for all the hosts to discover every target.
    • tgtadm_logicalunit_new(target_id, dev): Binds an LVM logical volumen to a target.
    • tgtadm_target_delete(target_id): Removes a target
    • iscsiadm_discovery(host): Issues iscsiadm discovery
    • iscsiadm_login(iqn, target_host): Logins to an already discovered IQN.
    • iscsiadm_logout(target_it): Logs out from an IQN.
    • is_iscsi(host): Returns 0 if logins/logouts should be performed on that host.
    • iqn_get_lv_name(iqn): Extracts the logical volume name which is encoded in the IQN.
    • iqn_get_vg_name(iqn): Extracts the volume group name which is encoded in the IQN.
    • iqn_get_host(iqn): Extracts the iSCSI target host which is encoded in the IQN.
  • datastore/iscsi/cp: Registers a new image. Creates a new logical volume under LVM and sets an iSCSI target for it.
  • datastore/iscsi/mkfs: Makes a new empty image. Creates a new logical volume under LVM and sets an iSCSI target for it.
  • datastore/iscsi/rm: Removes the iSCSI target and removes the logical volume.
  • tm/iscsi/ln: iscsiadm discover and logs in.
  • tm/iscsi/clone: Not implemented (no support for non-persistent images).
  • tm/iscsi/mvds: Logs out for shutdown, cancel, delete, stop and migrate.