The Linux SCSI Target Wiki
LIO Unified Target
|Original author(s)||Nicholas Bellinger|
|Initial release||January 14, 2011|
|Stable release||4.1.0 / June 20, 2012|
|Preview release||4.2.0-rc5 / June 28, 2012|
|License||GNU General Public License|
The Unified Target has become the new Linux standard with kernel version 2.6.38. LIO Unified Target supports a wide range of platforms (from PC architecture to mobile devices, STBs and game consoles), based on a wide range of CPU architectures (x86, ia64, Alpha, Cell, PPC, ARM, MIPS, etc.), a growing number of fabric modules, and basically all existing Linux block devices for backstores.
The Unified Target core is a SCSI engine that implements the semantics of a SCSI target as described in the SCSI Architecture Model (SAM), and supports its comprehensive SPC-3/SPC-4 feature set in a fabric-agnostic way. The target core does not directly communicate with initiators and it does not directly access data on disk.
Fabric modules implement the "frontend" of the target. They "speak" specific protocols that transport SCSI commands. The Fabric Hardware Abstraction Layer (F-HAL) allows all protocol-specific processing to be encapsulated in fabric modules. The following fabric modules are available:
- Fibre Channel: support for FC Host Bus Adapters (HBAs).
- Fibre Channel over Ethernet (FCoE): FC access over Ethernet (tcm_fc.ko, released).
- FireWire / iEEE 1394 (sbp.ko, released).
- IBM vSCSI: IBM virtual SCSI (vSCSI) module for IBM System p (ibmvscsis.ko, RC).
- InfiniBand: support for Infiniband Host Channel Adapters (HCAs).
- iSCSI: SCSI access over IP networks (iscsi.ko, released).
- tcm_loop: SCSI emulation device to serve any type of raw hardware to local applications and VMs as a fully SCSI SPC-4 compliant block device, including emulation of advanced functionality (loopback.ko, released).
- USB Gadget (tcm_usb_gadget.ko, released).
- vHost: QEMU virtio and virtio-scsi PV guests (tcm_vhost.ko, released).
Backstores implement the "backend" of the target. They implement the methods of accessing data on disk. A backstore subsystem plugin is a physical storage object that provides the block device underlying a SCSI Endpoint.
Backstore objects can be added via the Storage Hardware Abstraction Layer (S-HAL) that brings storage hardware into the Target engine as raw block devices, on which the Linux stack just works (including complex functionality such as software RAID, LVM, snapshots, virtualization, etc.).
The Target supports the SCSI-3 standard for all backstore devices (block devices and/or VFS):
- FILEIO (Linux VFS devices): any file on a mounted filesystem. It may be backed by a file or an underlying real block device. FILEIO is using struct file to serve block I/O with various methods (synchronous or asynchronous) and (buffered or direct). The Linux kernel code for filesystems resides in linux/fs. By default, FILEIO uses
- IBLOCK (Linux BLOCK devices): any block device that appears in /sys/block. The Linux kernel code for block devices is in linux/block.
- PSCSI (Linux pass-through SCSI devices): any storage object that supports direct pass-through of SCSI commands without SCSI emulation. This assumes an underlying SCSI device that appears with lsscsi in /proc/scsi/scsi, such as a SAS hard drive, etc. SCSI-3 and higher is supported with this subsystem, but only for control CDBs capable by the device firmware.
- Direct Mapped RAMDISK (Linux RAMDISK_DR): Direct mapped RAMDISK (rd_dr) devices provide some SCSI emulation for direct mapped RAM disks. This is is most useful for fast volatile mass storage for benchmarking.
- Memory Copy RAMDISK (Linux RAMDISK_MCP): Memory Copy RAMDISKS (rd_mcp) provide RAMDISKS with full SCSI emulation and separate memory mappings using memory copy for initiators, thus providing multi-session capability. This is most useful for fast volatile mass storage for production.
The SCSI functionality is implemented directly in the target engine in a fabric agnostic way, including a number of high-end features, such as Persistent Reservations (PRs) and Asymmetric Logical Unit Assignment (ALUA), following the SPC-4 standard.
The backstore devices (FILEIO, IBLOCK, pSCSI, RAMDISK, etc.) report the underlying HW limitiations for things like TCQ depth, MaxSectors, TaskAbortedStatus, UA Interlocking, etc. All of these values are available as attributes in the targetcli device context.
The following specifications are available as T10 Working Drafts:
- SCSI Architecture Model - 2 (SAM-2): SAM-2 describes the second generation of the SCSI Architecture Model. Status: Published, 9/11/2002
- SCSI Architecture Model - 3 (SAM-3): SAM-3 describes the third generation of the SCSI Architecture Model. Status: Published, 9/21/2004
- SCSI Architecture Model - 4 (SAM-4): SAM-4 describes the fourth generation of the SCSI Architecture Model. Status: Published, 5/8/2008
- SCSI Primary Commands - 2 (SPC-2): contains the second-generation definition of the basic commands for all SCSI devices. SPC-2 is used in conjuction with a standard for the specific device type. In 2006, the ANSI SPC-2 standard was replaced with ISO/IEC 14776-452:2005, which is available through ANSI. Status: Published, 7/18/2001
- SCSI Primary Commands - 3 (SPC-3): contains the third-generation definition of the basic commands for all SCSI devices. SPC-3 is used in conjuction with a standard for the specific device type. Status: Published, 5/4/2005
- SCSI Primary Commands - 4 (SPC-4): contains the fourth-generation definition of the basic commands for all SCSI devices. SPC-4 is used in conjuction with a standard for the specific device type. Status: Development, 11/16/2010
- Small Computer System Interface - 2 (SCSI-2): SCSI-2 defines the second generation of the Small Computer System Interface. Status: Published, 9/7/1993
- Access Control List (ACL): Used to specify the access rights for Initiators to TPGs.
- Backstore: A physical storage object that provides the actual storage underlying an SCSI Endpoint.
- Command Descriptor Block (CDB): The standard format for SCSI commands. CDBs are commonly 6, 10, or 12 bytes long, though they can be 16 bytes or of variable length.
- Challenge Handshake Authentication Protocol (CHAP): An authentication technique for confirming the identity of one computer to another. Described in RFC 1994.
- Connection Identifier (CID): A 16-bit number, generated by the Initiator, that uniquely identifies a connection between two iSCSI devices. This number is presented during the login phase.
- Endpoint: The combination of an iSCSI TargetName with an iSCSI TPG Tag (IQN + Tag).
- Demo Mode: Means disabling authentification for an iSCSI Endpoint, i.e. its ACLs are diabled. Demo Mode grants read-only access to all iSCSI Initiators that attempt to connect to that specific Endpoint. See the iSCSI entry on how to enable Demo Mode.
- Extended Unique Identifier (EUI): A 64-bit number that uniquely identifies every device in the world. The format consists of 24 bits that are unique to a given company, and 40 bits assigned by the company to each device it builds.
- I_T Nexus: An I_T Nexus denotes a live session between an Initiator and a Target.
- Initiator: The originating end of a SCSI session. Typically a controlling device such as a computer.
- Internet Protocol Storage (IPS): The class of protocols or devices that use the IP protocol to move data in a storage network. FCIP, iFCP, and iSCSI are all examples of IPS protocols.
- iSCSI Qualified Name (IQN): A name format for iSCSI that uniquely identifies every device in the world (e.g.
- Initiator Session Identifier (ISID): A 48-bit number, generated by the Initiator, that uniquely identifies a session between the Initiator and the Target. This value is created during the login process, and is sent to the target with a Login PDU.
- Multipath I/O (MPIO): A method by which data can take multiple redundant paths between a server and storage.
- Network Portal: The combination of an iSCSI Endpoint with an IP address plus a TCP port. The TCP port number for the iSCSI protocol defined by IANA is 3260.
- Organizationally Unique Identifier (OUI) is a 24-bit number that is purchased from the IEEE Registration Authority. This identifier uniquely identifies a vendor, manufacturer, or other organization (referred to by the IEEE as the “assignee”) globally or worldwide and effectively reserves a block of each possible type of derivative identifier (such as MAC addresses, group addresses, Subnetwork Access Protocol protocol identifiers, etc.) for the exclusive use of the assignee, see OUI Wikipedia entry. The OUI is subsequently used by the assignee to create particular instances of these identifiers for various purposes, such as the identification of a particular piece of equipment.
- SCSI Architectural Model (SAM): A document that describes the behavior of SCSI in general terms, allowing for different types of devices communicating over various media.
- Target: The receiving end of a SCSI session, typically a device such as a disk drive, solid state drive, tape drive, or scanner.
- Target Group: A collection of fabric module SCSI target endpoints containing SCSI target ports that provide access to individual storage objects.
- Target Port: The combination of an iSCSI Endpoint with one or more LUNs.
- Target Portal Group (TPG): A list of IP addresses and TCP port numbers that determines which interfaces a specific iSCSI target will listen to.
- Target Session Identifier (TSID): A 16-bit number, generated by the target, that uniquely identifies a session between the initiator and the target. This value is created during the login process, and is sent to the initiator with a Login Response PDU.
- World Wide Name (WWN) or World Wide Identifier (WWID) is a unique identifier which identifies a particular Fibre Channel, ATA or SAS target. Each WWN is an 8 byte number derived from an IEEE OUI and vendor-supplied information, see also WWN Wikipedia entry. There are two formats of WWN defined by the IEEE:
- Original format: addresses are assigned to manufacturers by the IEEE standards committee, and are built into the device at build time, similar to an Ethernet MAC address. The first 2 bytes are either hex 10:00 or 2x:xx (where the x's are vendor-specified) followed by the 3-byte vendor identifier and 3 bytes for a vendor-specified serial number
- New addressing schema: the first nibble is either hex 5 or 6 followed by a 3-byte vendor identifier and 36 bits for a vendor-specified serial number
Inclusion in Linux distributions
Target and targetcli are included in most Linux distributions per default. Here is an overview over the most popular distributions:
|Distribution||Version[Linux 1]||Release||Archive||Install||Source git[Linux 2]||Documentation|
|CentOS||6.2||2011-12-20||CentOS mirror||su -c 'yum install fcoe-target-utils'||targetcli-fb.git||Tech Notes|
|Debian||7.0 ("wheezy")||TBA||Debian pool||su -c 'apt-get install targetcli'||targetcli|
|Fedora||16, 17/18||2011-11-08||Fedora Rawhide||su -c 'yum install targetcli'||targetcli-fb.git||Target Wiki|
|openSUSE||12.1||2011-11-08||Requires manual installation from targetcli.|
|RHEL||6.2||2011-11-16||Fedora Rawhide||su -c 'yum install fcoe-target-utils'||targetcli-fb.git||Tech Notes|
|Scientific Linux||6.2||2012-02-16||SL Mirror||su -c 'yum install fcoe-target-utils'||targetcli-fb.git||Tech Notes|
|SLES||SP2||2012-02-15||Requires manual installation from targetcli.|
|Ubuntu||PrecisePangolin v12||2012-04-26||Ubuntu universe||su -c 'apt-get install targetcli'||targetcli|
- ↑ The distribution release where LIO was included first.
- ↑ Technical support, and qualified backports to other kernels and distributions are available from Datera.
|Timeline of the LinuxIO|
|Feature||LIO Core||Loop back||FCoE||iSCSI||Perf|| SRP||CM WQ|| FC|
|vHost||Perf||Misc||16 GFC||iSER||Misc||VAAI||Misc|| DIF Core|
|DIF iSER||DIF FC vhost||TCMU Xen||Misc||Misc||virtio 1.0||Misc||NVMe OF|
The target and fabric modules have gone upstream into the Linux kernel as follows:
- Linux 2.6.38 (3/14/2011): Generic multiprotocol target engine
- Linux 2.6.39 (5/18/2011): tcm_loop (SCSI support on top of any raw hardware)
- Linux 3.0 (7/21/2011): FCoE (by Cisco)
- Linux 3.1 (10/24/2011): iSCSI
- Linux 3.3 (3/18/2012): InfiniBand/SRP (Mellanox HCAs)
- Linux 3.5 (7/21/2012): Fibre Channel (QLogic HBAs), USB Gadget and IEEE 1394
- Linux 3.6 (confirmed): vHost (QEMU virtio and virtio-scsi PV guests)
- Linux 3.7 (planned): IBM vSCSI (for the IBM pSeries)
- RFC 1994: PPP Challenge Handshake Authentication Protocol (CHAP)
- Fabrics: ATA over Ethernet (AoE), FibreChannel over Ethernet (FCoE), FibreChannel (FC), InfiniBand (IB), iSCSI, iSCSI Extensions for RDMA (iSER), Internet Wide Area RDMA Protocol (iWARP), and SCSI RDMA Protocol (SRP)
- Services: Service Location Protocol (SLP), Internet Storage Name Service (iSNS)
- Other: Storage Area Network (SAN), Serial attached SCSI (SAS), World Wide Name (WWN)
- RTS OS, targetcli
- Fabric modules: iSCSI, Fibre Channel, FCoE, InfiniBand, IBM vSCSI, tcm_loop
- Distributions: RHEL 4/5/6, SLES11, CentOS, Debian, Fedora, openSUSE, Ubuntu, etc.
- Platforms: PC architecture (x86/ia64), PlayStation 2 and PlayStation 3 (Cell), etc.
- Low level kernel API: configFS
- Standards: SCSI
- ↑ Thorsten Leemhuis (3/2/2011). "Kernel Log: Coming in 2.6.38 (Part 4) - Storage". Heise Online.
- ↑ Jonathan Corbet (12/22/2010). "Shooting at SCSI targets". lnw.net.
- ↑ Linus Torvalds (2011-03-14). "Linux 2.6.38". lkml.org.
- ↑ Linus Torvalds (3/14/2011). "Linux 2.6.38". lkml.org.
- ↑ Linus Torvalds (1/14/2011). "Target merge". lkml.org.
- ↑ Linus Torvalds (5/18/2011). "Linux 2.6.39". lkml.org.
- ↑ Linus Torvalds (7/21/2011). "Linux 3.0". lkml.org.
- ↑ Linus Torvalds (10/24/2011). "Linux 3.1". lkml.org.
- ↑ Linus Torvalds (7/27/2011). "iSCSI merge". lkml.org.
- ↑ Linus Torvalds (3/18/2012). "Linux 3.3 release". lkml.org.
- ↑ Linus Torvalds (1/18/2012). "InfiniBand/SRP merge". lkml.org.
- ↑ Linus Torvalds (7/21/2012). "Linux 3.5 released". marc.info.
- ↑ Linus Torvalds (5/31/2012). "scsi-misc". lkml.org.
- ↑ Linus Torvalds (5/22/2012). "usb-target-merge". lkml.org.
- ↑ Linus Torvalds (5/23/2012). "sbp-target-merge". lkml.org.
- ↑ Linus Torvalds (8/13/2012). "tcm_vhost: Initial merge for vhost level target fabric driver". lkml.org.
- LIO Wikipedia entries: German English
- RTS OS Admin Manual
- RTSlib Reference Guide [HTML][PDF]
- RTS OS VAAI video. YouTube.
- Adapters by SCSI connector type
- Anatomy of the Linux SCSI subsystem
- Organizationally Unique Identifier (OUI) Wikipedia entry
- SCSI Wikipedia entry
- SCSI Tutorial
- SCSI/iSCSI/RAID/SAS Information Sheet
- T10 Technical Committee (SCSI standards)
- T10 Technical Committee - Protocols and Physical Layers
- T10 Primary Commands (SPC): using SPC-3/4
- T10 Block Commands (SBC) using SBC-3
- wetpaint.com Fun storage stuff (Ming Zhang)