SCST is a GPL licensed SCSI target software stack. The design goals of this software stack are high performance, high reliability, strict conformance to existing

SCSI Small Computer System Interface (SCSI, ) is a set of standards for physically connecting and transferring data between computers and peripheral devices, best known for its use with storage devices such as hard disk drives. SCSI was introduced ...

standards, being easy to extend and easy to use. SCST does not only support multiple SCSI protocols (

iSCSI Internet Small Computer Systems Interface or iSCSI ( ) is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP ...

, FC, SRP, ...) but also supports multiple local storage interfaces (SCSI pass-through,

block Block or blocked may refer to: Arts, entertainment and media Broadcasting * Block programming, the result of a programming strategy in broadcasting * W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...

I/O and file I/O) and also storage drivers implemented in user-space via the scst_user driver. In order to reach maximum performance SCST has been implemented as a set of kernel drivers. SCST is often combined with

RAID RAID (; redundant array of inexpensive disks or redundant array of independent disks) is a data storage virtualization technology that combines multiple physical Computer data storage, data storage components into one or more logical units for th ...

data deduplication In computing, data deduplication is a technique for eliminating duplicate copies of repeating data. Successful implementation of the technique can improve storage utilization, which may in turn lower capital expenditure by reducing the overall amou ...

and/or

high-availability cluster In computing, high-availability clusters (HA clusters) or fail-over clusters are groups of computers that support server applications that can be reliably utilized with a minimum amount of down-time. They operate by using high availability sof ...

software to augment its functionality. The SCST software stack is the basis software of many SAN systems. Several world records have been set with SAN systems based on SCST. SCST competes with

LIO Target The Linux-IO Target (LIO) is an open-source Small Computer System Interface (SCSI) target implementation included with the Linux kernel. Unlike initiators, which begin sessions, LIO functions as a target, presenting one or more Logical Unit ...

for the same purpose of providing a generic SCSI target module inside the Linux kernel. For the narrower purpose providing a Linux

iSCSI target Internet Small Computer Systems Interface or iSCSI ( ) is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP ...

, the older IET and STGT modules also enjoy industry support. Between 2013 and 2017 SCST was primarily maintained by SanDisk personnel, after

Fusion-io Fusion-io, Inc. was a computer hardware and software systems company (acquired by SanDisk Corporation in 2014) based in Cottonwood Heights, Utah, that designed and manufactured products using flash memory technology. The Fusion was marketed ...

acquired the start-up ID7 in 2013, a driving force behind SCST and after SanDisk acquired

in 2014.

Architecture

SCST consists of three groups of modules: * The SCST core, a protocol-independent engine for processing SCSI commands. * Target drivers which receive SCSI commands from a SCSI initiator, pass these SCSI to the SCST core and send back replies to the initiator. * Storage drivers a.k.a. device handlers which interact with the storage medium. The supported local storage interfaces are SCSI, block device, file and scst_user. scst_user is an SCST-specific protocol that allows efficient implementation of storage drivers in user space. Configuration of all these modules happens via a sysfs interface. Although direct configuration of SCST via its sysfs interface is convenient, the tool called scstadmin allows to control SCST via its sysfs interface and also to save and restore the SCST configuration. The following design aspects help SCST to reach the high performance this project is known for: * The number of threads per storage device that processes SCSI commands is configurable. * All target driver and storage driver functions invoked by the SCST core are asynchronous, at least when the Linux kernel allows this. It is e.g. not yet possible to perform asynchronous or direct I/O from inside the Linux kernel. * One

finite-state machine A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation. It is an abstract machine that can be in exactly one of a finite number o ...

is associated with each SCSI command. This allows a single command thread to process multiple SCSI commands concurrently. * Even when using multiple threads per storage device all these threads share the same I/O scheduler context. This sharing improves performance significantly when using e.g. the CFQ I/O scheduler. * A scatter-gather vector cache. This cache avoids having to allocate memory via the kernel for each SCSI command. * Lockless access to certain data structures needed to process SCSI commands. This also means that I/O has to be suspended before any of the data structures that are accessed without locking can be modified. * SCSI commands can be submitted by a target driver to the SCST core from softirq context. This results in a lower number of context switches compared to having to submit SCSI commands from thread context.

History

In the year 2000 a SCSI target framework was developed by the

University of New Hampshire InterOperability Laboratory The University of New Hampshire InterOperability Laboratory (UNH-IOL) is an independent test organization that provides interoperability and standards conformance testing for networking, telecommunications, data storage, and consumer technology ...

(UNH-IOL). Although the UNH-IOL SCSI target implementation only supports the

protocol, it was designed such that support for other storage protocols would be easy to add. In 2004 the SCST project was announced, providing

Fibre Channel Fibre Channel (FC) is a high-speed data transfer protocol providing in-order, lossless delivery of raw block data. Fibre Channel is primarily used to connect computer data storage to Server (computing), servers in storage area networks (SAN) in ...

and iSCSI support. The initial iSCSI target in SCST was based on the UNH-IOL iSCSI target implementation. Around that time an alternative iSCSI target software stack was developed by Ardis Technologies. The iSCSI Enterprise Target software, announced in 2005, is a further evolution of the Ardis stack. Later in 2005 Chelsio announced an SCST iSCSI target driver for the Chelsio T2 Ethernet adapter family. In 2006 a target driver for LSI/MPT cards was added. Later in this year the scstadmin tool was added. This tool allows to save and restore the SCST configuration from or to a file. In 2007 the UNH-IOL iSCSI target patch in the SCST project was replaced by a fork of IET. In 2008 an SRP target driver was donated by Mellanox. Later on this driver was optimized by Bart Van Assche. In 2009 Marvell donated a target driver for their SAS cards. Also in 2009 a target driver for Emulex

FCoE Fibre Channel over Ethernet (FCoE) is a computer network technology that encapsulates Fibre Channel frames over Ethernet networks. This allows Fibre Channel to use 10 Gigabit Ethernet networks (or higher speeds) while preserving the Fibre Channel ...

adapter cards was announced by Emulex. The same year a sysfs interface has been added to SCST next to the already existing procfs interface. In 2010 an

target driver was donated by Cisco and Bart Van Assche ported the

IBM System p The IBM System p is a high-end line of RISC ( Power)/UNIX-based servers. It was the successor of the RS/6000 line, and predecessor of the IBM Power Systems server series. History The previous RS/6000 line was originally a line of workstations ...

SCSI target driver to SCST. In 2011 implicit ALUA support was added to the SCST core and to the scstadmin configuration tool. The three init scripts (scst, iscsi-scst and qla2x00t) were merged into a single init script, simplifying installation and management of SCST. In 2013 support for QLogic InfiniBand HCA's was added in the SRP target driver. In January 2014 Mellanox donated an iSER target driver implementation to the SCST project. In May 2014 QLogic made a 16 Gbit/s QLogic target driver available and also announced to take over maintainership of the SCST QLogic target driver. In 2016 support for asynchronous buffered and direct file I/O and also a more powerful latency measurement infrastructure were added. In 2019 the SCST core was modified such that I/O keeps running at full speed if a LUN or storage device is added or removed.

References

{{reflist, 2

External links

Enterprise Storage OS
(ESOS) - An open source, high performance, block-level storage platform. * Bill Childers
OpenFiler: an Open-Source Network Storage Appliance
''Linux Journal'' issue 181, May 2009. * Errol Neal
SSD + ZFS/bcache/EnhanceIO + SCST = Hybrid Storage Array?
ha-guru.com, 22 July 2014. Free software programmed in C Linux kernel-related software SCSI Free storage software Software using the GNU General Public License

Architecture

History

See also

References

External links