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ISCSI

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Internet SCSI (iSCSI) is an official standard ratified on February 11 2003 by the Internet Engineering Task Force that allows the use of the SCSI protocol over TCP/IP networks. iSCSI is a transport layer protocol in the SCSI-3 specifications framework. Other protocols in the transport layer include SCSI Parallel Interface and Fibre Channel.

Acceptance of iSCSI in corporate production environments has accelerated now that Gigabit Ethernet is common. Building iSCSI-based Storage Area Networks (SAN) has become a less costly but worthy alternative to creating Fibre Channel-based SANs.

Contents

Functionality

The iSCSI protocol uses TCP/IP for its data transfer. Unlike other network storage protocols, such as Fibre Channel (which is the foundation of most SANs), it requires only the simple and ubiquitous Ethernet interface (or any other TCP/IP-capable network) to operate. This enables low-cost centralization of storage without all of the usual expense and incompatibility normally associated with Fibre Channel storage area networks.

Critics of iSCSI expect worse performance than Fibre Channel due to the protocol overhead TCP/IP adds to the communication between client and storage. However new techniques like TCP Offload Engine (TOE) help in reducing this overhead. Tests have shown excellent performance of iSCSI SANs, whether TOEs or plain Gigabit Ethernet NICs were used. In fact, in modern high-performance servers, a plain NIC with efficient network driver code can outperform a TOE card because fewer interrupts and DMA memory transfers are required. Initial iSCSI solutions are based on a software stack. The iSCSI market is growing steadily, and should improve in performance and usability as more organizations deploy Gigabit and 10 Gigabit networks, and manufacturers integrate iSCSI support into their operating systems, SAN products and storage subsystems. iSCSI becomes even more interesting since Ethernet is starting to support higher speeds than Fibre Channel.

Storage Devices

In the context of computer storage, iSCSI allows a machine to use an iSCSI initiator to connect to remote targets such as disks and tape drives on an IP network for block level I/O. From the point of view of the class drivers and application software, the devices appear as locally attached SCSI devices. More complex environments consisting of multiple hosts and/or devices are called Storage Area Networks (SAN).

iSCSI devices should not be confused with Network-Attached Storage (NAS) devices which include server software to handle the arbitration of simultaneous access requests from different hosts. Allowing multiple hosts to have simultaneous access to a single device is a difficult task common to all SCSI devices. Without host-to-host communication, each host is unaware of the state and intentions of the other hosts. This condition leads to nasty data corruption and race conditions. In the realm of disk storage, cluster software solves this issue.

iSCSI terminology and how iSCSI can be used by an OS

An iSCSI initiator, in client/server terminology, is akin to a client device that connects to some service offered by the server (in this case an iSCSI target). An iSCSI target is akin to a server, in that it provides block level access to its storage media (usually a hard drive, but can be other types of SCSI devices if the iSCSI target software supports them.) The primary difference between a regular client/server system and an iSCSI initiator/target system is that while many clients can be simultaneously served by a single server (many-to-one), only one iSCSI initiator can talk to a given iSCSI target at a time (one-to-one).

iSCSI can be used to give an OS block-level access to a remote device (one attached to another computer likely on the same LAN). Depending on the types of devices supported by the iSCSI target, this block-level access can be used to build RAID volumes using remote disks, or to connect and use remote DVD/CD-writers and scanners, or to simply access additional storage space. iSCSI devices are not shared by iSCSI amongst multiple computers at the same time; instead the OS in control of the iSCSI device is responsible for controlling remote access to the device. The initiator and the OS that it is running on will determine how the iSCSI device appears, and whether it is a /dev entry or gets a drive letter or something else.

Industry support

Initiators

OS Support Overview

OS First release date Version
AIX 10/2002 AIX 5.2
Windows 06/2003 2000, XP Pro, 2003
NetWare 08/2003 NetWare 6.5
HP-UX 10/2003 HP 11i v1, HP 11i v2
Solaris 02/2005 Solaris 10
Linux kernel 06/2005 2.6.12

Initiator Implementations

Software Initiators
iSCSI Host Bus Adapters
iSCSI host bus adapters (HBAs) are network interface controllers that incorporate a TCP Offload Engine with onboard iSCSI processing. iSCSI HBAs are usually abstracted purely as SCSI controllers by the operating system driver system. In this case, the HBA is not a part of the system network stack. Recently there has been considerable effort in providing standard vendor-independent iSCSI management facilities via operating system driver interfaces. These standard interfaces also work with software initiators.

Since iSCSI HBAs offload considerable processing in the iSCSI stack, interrupt generation is less than comparable software initiator setups. Applications leaving few spare CPU cycles should benefit from the use of a dedicated HBA.

iSCSI HBAs have the advantage of including PCI option ROMS to allow booting from iSCSI targets. Alternative iSCSI boot methods with software initiators would require substantial work.

The following vendors have developed iSCSI HBAs:

Targets

Most industry focus has been placed on creating iSCSI disk targets though iSCSI tape and medium changer targets are popular as well. So far, physical devices have not featured native iSCSI interfaces on a component level. Instead, devices with SCSI Parallel Interface or Fibre Channel interfaces are bridged by using iSCSI target software, external bridges, or controllers internal to the device enclosure.

Alternatively, disk and tape targets can be virtualizations. Rather than representing an actual physical device, an emulated virtual device is presented. The underlying implementation can deviate drastically from the presented target as is done with Virtual Tape Library (VTL) solutions. VTLs use disk storage for storing data written to virtual tapes. As with actual physical devices, virtual targets are presented by using iSCSI target software, external bridges, or controllers internal to the device enclosure.

Software-based Targets

Native iSCSI Targets

See also

External links

RFCs

 

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All text is available under the terms of the GNU Free Documentation License See Wikipedia Copyrights for details.

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