Linux RHCE - iSCSI - sharing disk resources

The iSCSI protocol is a typical client-server protocol. This tool allows you to expose a disk resource (target) to a client, which is called an initiator. iSCSI refers to the server providing virtual disks over the network as an iSCSI "target". The client mounting these disks is called an initiator, which makes sense considering that iSCSI commands originate from the client and the server responds to them. In an iSCSI relationship, separate software components are responsible for the implementation of the target and the initiator. All modern operating systems include an initiator (though it is sometimes an optional component), and most systems already have a built-in target implementation. The iSCSI standard specification is defined in document RFC 3720. 

There were at least four different iSCSI initiator implementations for Linux environments. Several died a natural death, while others were merged. The only one that has survived to this day appears to be Open-iSCSI, the standard initiator available for all Linux distributions. To use it, we install the open-iscsi package on Ubuntu or iscsi-initiator-utils on RedHat systems. 

Step 3 - Setting the service to start on system boot/reboot 

Step 4 - Checking startup settings 

The tgtadm tool along with its available switches is used to configure the sharing. 

A list of switches can be found in the system manual and can be accessed by running the command: 

man tgtadm

CONFIGURING THE SHARED RESOURCE 

Step 1 - Creating a target 

Step 2 - Verifying that the new target has been created 

As shown above, the new target has been created and already contains a new Logical Unit - LUN 0, which is used to manage the target. You can also see the assigned name (nowy_target) and the iscsi driver used. 

Step 3 - Creating a new custom Logical Unit (LUN)

# tgtadm --lld iscsi --op new --mode logicalunit --tid 1 --lun 1 --backing-store /dev/sdb2

Step 4 - Verifying the creation of the new LUN 

BASIC SECURITY SETTINGS

When checking and displaying information about the created target and LUN, the last line contains: ACL information, which defines access rights to the created LUN. 

Step 1 - Restricting resource sharing to a selected host 

# tgtadm --lld iscsi --op bind --mode target --tid 1 --initiator-address 192.168.56.101

Step 2 - Verifying the changes made 

CONFIGURING SYSTEM FIREWALL - IPTABLES 

Step 1 - Checking which port the tgtd service is running on 

Step 2 - Adding appropriate rules to the iptables system firewall 

# iptables -I INPUT 16  -p tcp -m tcp --dport 3260 -j ACCEPT

After performing the steps above, the selected resource is shared. In our case, it was /dev/sdb2.

SAVING THE CONFIGURATION 

If all the steps above have been performed correctly, it would be useful to enable sharing after a system reboot. The target configuration is located in the file: 

Step 1 - Generating current information about the configured target.

This information must be included in /etc/tgt/targets.conf, so you can redirect it immediately 

# tgt-admin --dump > /etc/tgt/targets.conf

CONFIGURING THE INITIATOR - CLIENT 

Step 1 - Installing the required packages

# yum install -y iscsi-initiator-utils.i686

Step 2 - Verifying package installation 

Step 3 - Setting the service to start on system boot 

NOTE

The services will be started provided that the target to be mounted has been set beforehand. 

After installation, the services are configured to be started with the system

DISCOVERING THE SHARED TARGET 

Step 1 - Discovering the shared resource 

As before, the system manual will be useful here, where we will find all the necessary commands: 

Step 2 - Logging in to the new resource by providing the target name and host address

Step 3 - Verifying the connection of the new resource 

If you want to disconnect the resource, run the command:


# iscsiadm --mode node --targetname nowy_target --portal 192.168.56.102:3260 --logout
Logging out of session [sid: 1, target: nowy_target, portal: 192.168.56.102,3260]
Logout of [sid: 1, target: nowy_target, portal: 192.168.56.2,3260] successful.