One of the central tasks in computer centers and when operating servers is providing hard disk capacity for server systems. Fibre Channel is often used for this purpose. iSCSI (Internet SCSI) solutions provide a lower-cost alternative to Fibre Channel that can leverage commodity servers and Ethernet networking equipment. Linux iSCSI provides iSCSI initiator and target software for connecting Linux servers to central storage systems.
Figure 14-1 iSCSI SAN with an iSNS Server
iSCSI is a storage networking protocol that facilitates data transfers of SCSI packets over TCP/IP networks between block storage devices and servers. iSCSI target software runs on the target server and defines the logical units as iSCSI target devices. iSCSI initiator software runs on different servers and connects to the target devices to make the storage devices available on that server.
IMPORTANT:It is not supported to run iSCSI target software and iSCSI initiator software on the same server in a production environment.
The iSCSI target and initiator servers communicate by sending SCSI packets at the IP level in your LAN. When an application running on the initiator server starts an inquiry for an iSCSI target device, the operating system produces the necessary SCSI commands. The SCSI commands are then embedded in IP packets and encrypted as necessary by software that is commonly known as the iSCSI initiator. The packets are transferred across the internal IP network to the corresponding iSCSI remote station, called the iSCSI target.
Many storage solutions provide access over iSCSI, but it is also possible to run a Linux server that provides an iSCSI target. In this case, it is important to set up a Linux server that is optimized for file system services. The iSCSI target accesses block devices in Linux. Therefore, it is possible to use RAID solutions to increase disk space as well as a lot of memory to improve data caching. For more information about RAID, also see Section 8.0, Software RAID Configuration.