iSCSI

What Is iSCSI Storage?

iSCSI（Internet Small Computer System Interface） is a storage area networking protocol that defines how data is transferred between host systems and storage devices. It enables block-level SCSI data transport between the iSCSI initiator and the storage target over TCP/IP networks. iSCSI storage is also known as iSCSI SAN（Storage Area Network） storage, which is a cost-effective alternative to traditional Fibre Channel (FC) SAN. Unlike FC SAN, which requires dedicated hardware (FC host bus adapter, FC switch, etc.), iSCSI SAN storage can be built on existing network infrastructure (Ethernet switch, router, etc.), rather than requiring us to buy additional hardware. Furthermore, iSCSI is based on block transfer standard, which is more fast and efficient.

How Does iSCSI Storage Work?

To better understand how iSCSI works, some vital iSCSI components such as iSCSI initiator and iSCSI target should be the first to learn. iSCSI initiator is a piece of software or hardware that is installed in a server to send requests to and receive responses from an iSCSI target. iSCSI target is on storage devices providing the storage, which listens and responds to commands from iSCSI initiators. The initiator resides on the client machine. It can be initiated by either software or hardware, but software initiation is much more common. Servers are the iSCSI target and can receive initiator requests on one or more TCP/IP connections.

As shown below, iSCSI storage works by transporting block-level data between iSCSI initiator on a server and iSCSI target on a storage device through the TCP/IP network. The iSCSI protocol encapsulates SCSI commands and assembles the data in packets for the TCP/IP layer. Packets are sent over the network using a point-to-point connection. Upon arrival, the iSCSI protocol disassembles the packets, separating the SCSI commands so the operating system will see the storage as if it was a locally connected SCSI device that can be formatted as usual.

Benefits of iSCSI

iSCSI provides a variety of advantages for enterprises implementing it as part of their SAN infrastructure. The most notable benefits include:

• TOE Integration: Network interface cards (NICs) come with built-in TCP offload engine (TOE) technology, which offloads the TCP/IP stack processing to a network controller, thereby reducing the processing load on the host CPU.

• Affordable Initial Setup: The iSCSI protocol allows businesses to maintain a low total cost of ownership (TCO) when accessing storage devices. The resulting SAN is not only cost-effective but also easy to install and maintain.

• No Additional Hardware Necessity: iSCSI operates on IP-based technology, leveraging the interoperability of TCP/IP and Ethernet without requiring any extra hardware.

• Enhanced Security: iSCSI adds an additional security layer to storage networks, bolstering data protection.

• Minimal Training and Maintenance Costs: iSCSI doesn’t demand extensive training for network staff, and it facilitates increased storage usage with minimal oversight, keeping operational costs low.

• Broad Compatibility: As a universal, non-proprietary technology, IP ensures seamless integration across various network storage equipment.

• Scalable Flexibility: iSCSI SAN environments can be easily expanded using existing network hardware, enabling businesses to scale their storage capacity as needed.

• Reduced Power Consumption and Costs: With fewer active switches and a streamlined infrastructure, iSCSI helps reduce power usage, cooling expenses, and overall infrastructure costs.

Limitations of iSCSI

While iSCSI offers several advantages, it does have its limitations. Here are the most notable ones:

• Performance Dependency: In networks that handle both business data and iSCSI traffic, iSCSI can lead to increased latency and performance degradation.

• Incompatibility with SSDs: Since iSCSI relies on serial technology, it is not well-suited for solid-state storage devices (SSDs).

• Capacity Constraints: The maximum capacity for a hard disk drive LUN is limited to 250 TB, and for NAS, the limit is 144 TB.

• Limited Snapshots: iSCSI applications can support up to 512 snapshots (virtual copies of data) and only 64 snapshots for file share volumes. Failure of the snapshot feature can result in data loss and adversely affect iSCSI performance.

• Connection Limits: iSCSI sessions are capped at 255 connections. Exceeding this limit will result in the target rejecting additional connection attempts.