iSCSI is an implementation of the Small Computer System Interface (SCSI) command set using the Internet Protocol (IP), specified in RFC 3720. iSCSI is implemented as a service where a client (the initiator) talks to a server (the target) via a session on TCP port 3260. An iSCSI target's IP address and port are called an iSCSI portal, where a target can be exposed through one or more portals. The combination of a target and one or more portals is called the target portal group (TPG).
The underlying data link layer protocol for iSCSI is commonly Ethernet. More specifically, modern iSCSI infrastructures use 10 Gigabit Ethernet or faster networks for optimal throughput. 10 Gigabit Ethernet connectivity between the iSCSI gateway and the back-end Ceph cluster is strongly recommended.
The Linux kernel iSCSI target was originally named LIO for linux-iscsi.org, the project's original domain and Web site. For some time, no fewer than 4 competing iSCSI target implementations were available for the Linux platform, but LIO ultimately prevailed as the single iSCSI reference target. The mainline kernel code for LIO uses the simple, but somewhat ambiguous name "target", distinguishing between "target core" and a variety of front-end and back-end target modules.
The most commonly used front-end module is arguably iSCSI. However, LIO also supports Fibre Channel (FC), Fibre Channel over Ethernet (FCoE) and several other front-end protocols. At this time, only the iSCSI protocol is supported by SUSE Enterprise Storage.
The most frequently used target back-end module is one that is capable of simply re-exporting any available block device on the target host. This module is named iblock. However, LIO also has an RBD-specific back-end module supporting parallelized multipath I/O access to RBD images.
This section introduces a brief information on iSCSI initiators used on Linux, Microsoft Windows, and VMware platforms.
The standard initiator for the Linux platform is open-iscsi. open-iscsi launches a daemon, iscsid, which the user can then use to discover iSCSI targets on any given portal, log in to targets, and map iSCSI volumes. iscsid communicates with the SCSI mid layer to create in-kernel block devices that the kernel can then treat like any other SCSI block device on the system. The open-iscsi initiator can be deploying in conjunction with the Device Mapper Multipath (dm-multipath) facility to provide a highly available iSCSI block device.
The default iSCSI initiator for the Microsoft Windows operating system is the Microsoft iSCSI initiator. The iSCSI service can be configured via a graphical user interface (GUI), and supports multipath I/O for high availability.
The default iSCSI initiator for VMware vSphere and ESX is the VMware ESX software iSCSI initiator, vmkiscsi. When enabled, it can be configured either from the vSphere client, or using the vmkiscsi-tool command. You can then format storage volumes connected through the vSphere iSCSI storage adapter with VMFS, and use them like any other VM storage device. The VMware initiator also supports multipath I/O for high availability.