What Is iSCSI and How Does It Work?

iSCSI stands for Internet Small Computer Systems Interface. iSCSI is a transport layer protocol that works on top of the Transport Control Protocol (TCP). It enables block-level SCSI data transport between the iSCSI initiator and the storage target over TCP/IP networks. iSCSI supports encrypting the network packets, and decrypts upon arrival at the target.

SCSI is a block-based set of commands that connects computing devices to networked storage, including spinning up storage media and data reads/writes.

The protocol uses initiators to send SCSI commands to storage device targets on remote servers. Storage targets may be SAN, NAS, tape, general-purpose servers – both SSD and HDD – LUNs, or others. The protocol allows admins to better utilize shared storage by allowing hosts to store data to remote networked storage, and virtualizes remote storage for applications that require direct attached storage.

iSCSI Performance

iSCSI performance is highly dependent on underlying technologies like 10 Gigabit Ethernet (10 GbE) and bridging technology in the data center.

  • 10 GbE. Ethernet network connection speed has the single largest impact on iSCSI performance. Although smaller networks may run iSCSI protocols over 1 GbE networks, the slower speed is insufficient for mid-sized or enterprise data centers. Admins may increase some performance on a sub-10 GbE network by adding multiple NICs, but a single switch will not boost speed for multiple iSCSI ports. 10 GbE is the recommended speed for an enterprise storage environment. Because it is a wider pipe, there is little call for multiple NICs. Instead, adding server-class network adapters will accelerate iSCSI packets traveling the 10 GbE network.
  • Data center bridging. Bridging is a set of Ethernet extensions that protect SCSI traffic against data loss. This allows iSCSI to better compete with highly reliable Fiber Channel, which has run over lossless connections for years.
  • Multipathing. Multipathing I/O speeds up iSCSI network packets, and most operating systems support the technology. Typical iSCSI multipathing features assign multiple addresses to a single iSCSI session, which accelerates data transport.
  • Jumbo frames. These 9000-byte frames relieve congestion on slower Ethernet networks that are not using 10 GbE, which gives a performance boost of about 10-20 percent. Jumbo frames will not give much of a performance boost in 10 GbE, if any.
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