Internet Small Computer System Interface (iSCSI) is a protocol that allows computers to communicate with storage devices by SCSI requests and responses carried over TCP/IP. Because iSCSI is based on the standard SCSI protocols, it uses some terminology from SCSI. The device on the SCSI bus to which requests get sent (and which answers these requests) is known as the target and the device issuing requests is known as the initiator. In other words, an iSCSI disk is a target and the iSCSI software equivalent of a SCSI controller or SCSI Host Bus Adapter (HBA) is called an initiator. This appendix only covers Linux as an iSCSI initiator: how Linux uses iSCSI disks, but not how Linux hosts iSCSI disks.
Linux has a software iSCSI initiator in the kernel that takes the place and form of a SCSI HBA driver and therefore allows Linux to use iSCSI disks. However, as iSCSI is a fully network-based protocol, iSCSI initiator support needs more than just the ability to send SCSI packets over the network. Before Linux can use an iSCSI target, Linux must find the target on the network and make a connection to it. In some cases, Linux must send authentication information to gain access to the target. Linux must also detect any failure of the network connection and must establish a new connection, including logging in again if necessary.
The discovery, connection, and logging in is handled in userspace by the iscsiadm utility, and the error handling is also handled in userspace by iscsid.
Both iscsiadm and iscsid are part of the iscsi-initiator-utils package under Red Hat Enterprise Linux.
B.1. iSCSI disks in anaconda
Anaconda can discover (and then log in to) iSCSI disks in two ways:
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When anaconda starts, it checks if the BIOS or add-on boot ROMs of the system support iSCSI Boot Firmware Table (iBFT), a BIOS extension for systems which can boot from iSCSI. If the BIOS supports iBFT, anaconda will read the iSCSI target information for the configured boot disk from the BIOS and log in to this target, making it available as an installation target.
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If you select the
Specialized Storage Devices option during installation, the storage device selection screen presents you with an
Add Advanced Target button. If you click this button, you can add iSCSI target information like the discovery IP address.
Anaconda probes the given IP address and logs in to any targets that it finds. See
Section 9.5.1.1, “ Advanced Storage Options ” for the details that you can specify for iSCSI targets.
While anaconda uses iscsiadm to find and log into iSCSI targets, iscsiadm automatically stores any information about these targets in the iscsiadm iSCSI database. Anaconda then copies this database to the installed system and marks any iSCSI targets not used for / so that the system will automatically log in to them when it starts. If / is placed on an iSCSI target, initrd will log into this target and anaconda does not include this target in start up scripts to avoid multiple attempts to log into the same target.
If / is placed on an iSCSI target, anaconda sets NetworkManager to ignore any network interfaces that were active during the installation process. These interfaces will also be configured by initrd when the system starts. If NetworkManager were to reconfigure these interfaces, the system would lose its connection to /.
