iSCSI Initiators: Accessing Networked Block Storage over Ethernet

Understand the role of iSCSI initiators in accessing networked block storage. This tutorial explains iSCSI technology, the communication between initiators and targets, and how iSCSI enables efficient access to storage devices over standard Ethernet networks using TCP/IP.

iSCSI Initiators: Accessing Networked Block Storage

Introduction to iSCSI

iSCSI (Internet Small Computer System Interface) is a networking technology that lets computers access block-level storage (like hard drives) over a standard Ethernet network using TCP/IP. This means you can connect to storage devices as if they were directly attached, even if they are located remotely. It's often called "SAN over IP" because it allows you to build a Storage Area Network (SAN) using your existing IP network infrastructure.

iSCSI Initiators and Targets

iSCSI communication involves two key components:

  • Initiator: The client device (computer, server) requesting access to storage. It can be implemented in software (a software driver) or hardware (a dedicated network card).
  • Target: The storage device (or server providing access to the storage) that responds to the initiator's requests. Targets can also be implemented in software or hardware.

Both initiators and targets are identified using unique names (often IQN - iSCSI Qualified Name).

How iSCSI Works

iSCSI uses a client-server model. The initiator sends SCSI commands (the standard language for talking to storage devices) to the target over TCP/IP. The target processes these commands and sends back responses. This communication happens using Protocol Data Units (PDUs), which are the basic units of data transfer in iSCSI.

Hardware vs. Software iSCSI Initiators

  • Hardware iSCSI Initiators: Dedicated network cards with built-in iSCSI and TCP/IP processing. These are often used when features like encryption or advanced security are required.
  • Software iSCSI Initiators: Software drivers or applications within the operating system that translate SCSI commands into TCP/IP commands. These are more common due to their flexibility and lower cost.

Setting Up an iSCSI Initiator

Setting up an iSCSI initiator varies by operating system:

Windows:

  1. Open the iSCSI Initiator application (search in the Start menu).
  2. Go to the "Targets" tab.
  3. Enter the target's IP address and click "Quick Connect".
  4. If necessary, refresh the "Discovery" tab to find the target.
  5. Follow the on-screen instructions to complete the setup.

Red Hat Enterprise Linux (RHEL):

  1. Install iscsi-initiator-utils.
  2. Verify the initiator name (cat /etc/iscsi/initiatorname.iscsi).
  3. Discover and connect to the target using its IQN.
  4. Create a filesystem and mount the iSCSI disk.
  5. Configure automatic mounting at boot (/etc/fstab).

Ubuntu:

  1. Install the open-iscsi package (sudo apt install open-iscsi).
  2. Configure /etc/iscsi/iscsid.conf and restart the daemon.
  3. Discover and configure automatic login to the target.
  4. Enable services at startup.

Benefits of Using iSCSI

  • Built-in TOE (TCP Offload Engine): Reduces CPU load on the server.
  • Low Initial Investment: Cost-effective compared to Fibre Channel SANs.
  • Leverages Existing Infrastructure: Uses standard Ethernet and TCP/IP.
  • Enhanced Security: Can incorporate encryption.
  • Low Maintenance: Requires minimal ongoing administration.
  • Wide Compatibility: Works with various storage devices.
  • Scalability: Easy to increase storage capacity.
  • Reduced Power and Cooling Costs: Fewer active network devices.

Limitations of iSCSI

  • Performance Dependency: Performance relies on network conditions.
  • Not Ideal for SSDs: May not fully leverage the speed of SSDs.
  • Capacity Limitations: Maximum LUN (Logical Unit Number) sizes are defined.
  • Limited Snapshots: Maximum numbers of snapshots are defined.
  • Connection Limits: Maximum number of simultaneous connections is restricted.