iSCSI
This chapter provides a high level overview of the iSCSI protocol.
The following topics are covered:
28.1 What iSCSI is
The iSCSI protocol is a licensed service on the storage system that enables you to transfer block data to hosts using the SCSI protocol over TCP/IP. The iSCSI protocol standard is defined by RFC 3720.
In an iSCSI network, storage systems are targets that have storage target devices, which are referred to as LUNs (logical units). A host with an iSCSI host bus adapter (HBA), or running iSCSI initiator software, uses the iSCSI protocol to access LUNs on a storage system. The iSCSI protocol is implemented over the storage system’s standard gigabit Ethernet interfaces using a software driver.
The connection between the initiator and target uses a standard TCP/IP network. No special network configuration is needed to support iSCSI traffic. The network can be a dedicated TCP/IP network, or it can be your regular public network. The storage system listens for iSCSI connections on TCP port 3260.
In an iSCSI network, there are two types of nodes: targets and initiators. Targets are storage systems, and initiators are hosts. Switches, routers, and ports are TCP/IP devices only, and are not iSCSI nodes.
Storage systems and hosts can be direct-attached through FC or connected through a TCP/IP network.
iSCSI can be implemented on the host using hardware or software. You can implement iSCSI in one of the following ways:
•Initiator software that uses the host’s standard Ethernet interfaces.
•An iSCSI host bus adapter (HBA): An iSCSI HBA appears to the host operating system as a SCSI disk adapter with local disks.
•TCP Offload Engine (TOE) adapter that offloads TCP/IP processing. The iSCSI protocol processing is still performed by host software.
You can implement iSCSI on the storage system using software solutions.
Target nodes can connect to the network in the following ways:
•Over the system's Ethernet interfaces using software that is integrated into Data ONTAP. iSCSI can be implemented over multiple system interfaces, and an interface used for iSCSI can also transmit traffic for other protocols, such as CIFS and NFS.
•On the 20xx, 30xx, and 60xx systems, using an iSCSI target expansion adapter, to which some of the iSCSI protocol processing is offloaded. You can implement both hardware-based and software-based methods on the same system.
•Using a unified target adapter (UTA).
28.2 How iSCSI nodes are identified
Every iSCSI node must have a node name.
The two formats, or type designators, for iSCSI node names are iqn and eui. The storage system always uses the iqn-type designator. The initiator can use either the iqn-type or eui-type designator.
28.2.1 The iqn-type designator
The iqn-type designator is a logical name that is not linked to an IP address. It is based on the following components:
•The type designator, such as iqn
•A node name, which can contain alphabetic characters (a to z), numbers (0 to 9), and three special characters:
– Period (“.”)
– Hyphen (“-”)
– Colon (“:”)
•The date when the naming authority acquired the domain name, followed by a period
•The name of the naming authority, optionally followed by a colon (:)
•A unique device name
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Tip: Some initiators might provide variations on the preceding format. Also, even though some hosts do support underscores in the host name, they are not supported on N series systems. For detailed information about the default initiator-supplied node name, see the documentation provided with your iSCSI Host Utilities.
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An example format is given in Example 28-1.
Example 28-1 The iSCSI format
iqn.yyyymm.backward naming authority:unique device name
yyyy-mm is the month and year in which the naming authority acquired the domain name.
backward naming authority is the reverse domain name of the entity responsible for naming this device. An example reverse domain name is com.microsoft.
unique-device-name is a free-format unique name for this device assigned by the naming authority.
The following example shows the iSCSI node name for an initiator that is an application server: iqn.1991-05.com.microsoft:example
28.2.2 Storage system node name
Each storage system has a default node name based on a reverse domain name and the serial number of the storage system's non-volatile RAM (NVRAM) card.
The node name is displayed in the following format:
iqn.1992-08.com.ibm:sn.serial-number
The following example shows the default node name for a storage system with the serial number 12345678:
iqn.1992-08.com.ibm:sn.12345678
28.2.3 The eui-type designator
The eui-type designator is based on the type designator, eui, followed by a period, followed by sixteen hexadecimal digits.
A format example is as follows: eui.0123456789abcdef
28.3 How the storage system checks initiator node names
The storage system checks the format of the initiator node name at session login time. If the initiator node name does not comply with storage system node name requirements, the storage system rejects the session.
28.4 Default port for iSCSI
The iSCSI protocol is configured in Data ONTAP to use TCP port number 3260.
Data ONTAP does not support changing the port number for iSCSI. Port number 3260 is registered as part of the iSCSI specification and cannot be used by any other application or service.
28.5 What target portal groups are
A target portal group is a set of network portals within an iSCSI node over which an iSCSI session is conducted.
In a target, a network portal is identified by its IP address and listening TCP port. For storage systems, each network interface can have one or more IP addresses and therefore one or more network portals. A network interface can be an Ethernet port, virtual local area network (VLAN), or interface group.
The assignment of target portals to portal groups is important for two reasons:
•The iSCSI protocol allows only one session between a specific iSCSI initiator port and a single portal group on the target.
•All connections within an iSCSI session must use target portals that belong to the same portal group.
By default, Data ONTAP maps each Ethernet interface on the storage system to its own default portal group. You can create new portal groups that contain multiple interfaces.
You can have only one session between an initiator and target using a given portal group. To support some multipath I/O (MPIO) solutions, you need to have separate portal groups for each path. Other initiators, including the Microsoft iSCSI initiator version 2.0, support MPIO to a single target portal group by using different initiator session IDs (ISIDs) with a single initiator node name.
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Tip: Although this configuration is supported, it is not advised for N series storage systems. For more information, see the technical report on iSCSI multipathing.
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28.6 What iSNS is
The Internet Storage Name Service (iSNS) is a protocol that enables automated discovery and management of iSCSI devices on a TCP/IP storage network. An iSNS server maintains information about active iSCSI devices on the network, including their IP addresses, iSCSI node names, and portal groups.
You can obtain an iSNS server from a third-party vendor. If you have an iSNS server on your network, and it is configured and enabled for use by both the initiator and the storage system, the storage system automatically registers its IP address, node name, and portal groups with the iSNS server when the iSNS service is started. The iSCSI initiator can query the iSNS server to discover the storage system as a target device.
If you do not have an iSNS server on your network, you must manually configure each target to be visible to the host.
Currently available iSNS servers support different versions of the iSNS specification. Depending on which iSNS server you are using, you might have to set a configuration parameter in the storage system.
28.7 What CHAP authentication is
The Challenge Handshake Authentication Protocol (CHAP) enables authenticated communication between iSCSI initiators and targets. When you use CHAP authentication, you define CHAP user names and passwords on both the initiator and the storage system.
During the initial stage of an iSCSI session, the initiator sends a login request to the storage system to begin the session. The login request includes the initiator’s CHAP user name and CHAP algorithm. The storage system responds with a CHAP challenge. The initiator provides a CHAP response. The storage system verifies the response and authenticates the initiator. The CHAP password is used to compute the response.
28.8 How iSCSI communication sessions work
During an iSCSI session, the initiator and the target communicate over their standard Ethernet interfaces, unless the host has an iSCSI HBA or a CNA.
The storage system appears as a single iSCSI target node with one iSCSI node name. For storage systems with a MultiStore license enabled, each vFiler unit is a target with a different iSCSI node name.
On the storage system, the interface can be an Ethernet port, interface group, UTA, or a virtual LAN (VLAN) interface.
Each interface on the target belongs to its own portal group by default. It enables an initiator port to conduct simultaneous iSCSI sessions on the target, with one session for each portal group. The storage system supports up to 1,024 simultaneous sessions, depending on its memory capacity. To determine whether your host’s initiator software or HBA can have multiple sessions with one storage system, see your host OS or initiator documentation.
You can change the assignment of target portals to portal groups as needed to support multi- connection sessions, multiple sessions, and multipath I/O.
Each session has an Initiator Session ID (ISID), a number that is determined by the initiator.
28.9 How iSCSI works with HA pairs
HA pairs provide high availability because one system in the HA pair can take over if its partner fails. During failover, the working system assumes the IP addresses of the failed partner and can continue to support iSCSI LUNs.
The two systems in the HA pair must have identical networking hardware with equivalent network configurations. The target portal group tags associated with each networking interface must be the same on both systems in the configuration. This ensures that the hosts see the same IP addresses and target portal group tags whether connected to the original storage system or connected to the partner during failover.
28.10 Further information
More details on the iSCSI protocol can be found in the Redbooks publication, IP Storage Networking: IBM NAS and iSCSI Solutions, SG24-6240, which is located at the following website:
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