What is iSCSI?

The SNIA dictionary defines Internet Small Computer Systems Interface (iSCSI) as a transport protocol that provides for the SCSI protocol to be carried over a TCP-based IP network, standardized by the Internet Engineering Task Force and described in RFC 3720.

iSCSI is a block protocol for storage networking and runs the very common SCSI storage protocol across a network connection which is usually Ethernet.

iSCSI, like Fibre Channel, can be used to create a Storage Area Network (SAN). iSCSI traffic can be run over a shared network or a dedicated storage network. However, iSCSI does not support file access Network Attached Storage (NAS) or object storage access (they use different transport protocols).

There are multiple transports that can be used for iSCSI. The most common is TCP/IP over Ethernet, but Remote Direct Memory Access (RDMA) can also be used with iSER, which is iSCSI Extensions for RDMA. If using iSER, the transport is RoCE or InfiniBand and the underlying network is Ethernet (for RoCE) or InfiniBand (for InfiniBand transport).

iSCSI offers good block storage performance along with low cost. It is also widely supported by all major operating systems and hypervisors and can run on standard network cards or specialized Host Bus Adapters (HBAs). It is also supported by almost all enterprise storage arrays. For these reasons it has been popular for so-called “Tier 2” applications that require good, but not the best, block storage performance, and for storage that is shared by many hosts. It also is very popular among hyperscalers and large cloud service providers when they need a block storage solution that runs over Ethernet.

iSCSI can also be accelerated by using network adapters with an iSCSI hardware offload and/or a TCP Offload Engine (TOE). In the former, the hardware adapter (or HBA) offloads the iSCSI initiator function from the server CPU. In the latter case, the adapter offloads the TCP processing from the server kernel and CPU.

The rapid growth of faster Ethernet speeds such as 25G, 50G and 100G, along with increasing support for congestion management and traffic quality of service (QoS) on Ethernet switches, has greatly improved the potential performance, reliability and predictability of iSCSI as a storage protocol.


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