The iSCSI protocol is a storage area network (SAN) protocol which allows iSCSI initiators to use storage devices on the (remote) iSCSI target using normal ethernet cabling. To the iSCSI initiator, the remote storage looks like a normal, locally-attached hard drive.
What is iSCSI ?
It is a network storage protocol above TCP/IP. This protocol encapsulates SCSI data into TCP packets. iSCSI allows us to connect a host to a storage array via a simple Ethernet connection (tape drive). This solution is cheaper than the Fibre Channel SAN (Fibre channel HBAs and switches are expensive). From the host view the user sees the storage array.
LUN
LUNs like a local disks. iSCSI devices should not be confused with the NAS devices (for example NFS). The most important difference is that NFS volumes can be accessed by multiple hosts, but one iSCSI volume can by accessed by one host. It is similar to SCSIi protocol: usually only one host has access to one SCSI disk (the difference is the cluster enviroment). The iSCSI protocol is defined in the RFC3720 document by the IETF (Internet Engineering Task Force).
Some critics said that iSCSI has a worse performance comparing to Fibre Channel and causes high CPU load at the host machines. I think if we use Gigabit ethernet, the speed can be enough. To overcome the high CPU load, some vendors developed the iSCSI TOE-s (TCP Offload Engine). It means that the card has a built in network chip, which creates and computes the tcp frames. The Linux kernel doesn’t support directly this and the card vendors write their own drivers for the OS.
Initiator
The initiator is the name of the iSCSI client. The iSCSI client has a block level access to the iSCSI target. One client can use multiple iSCSI targets.
Target
The target is the name of the iSCSI server.
Discovery
Discovery is the process which shows the targets for the initiator.
