Generic SCSI Target Middle Level for Linux

The SCSI target mid-level subsystem for Linux (SCST) is a new subsystem of the Linux kernel that provides a standard framework for SCSI target drivers development. It is designed to provide unified, consistent interface between SCSI target drivers and Linux kernel and simplify target drivers development as much as possible. A system with a SCSI target device is able to share its local or virtual devices with other systems on a network with SCSI protocol support, e.g. SCSI bus, Fibre Channel, TCP/IP with iSCSI. This is commonly used for data storage virtualization.

SCST has the following features:

• Simple, easy to use interface with target drivers. Particularly, SCST performs all required pre- and post- processing of incoming requests and all necessary error recovery.
• Undertakes most problems, related to execution contexts, thus practically eliminating one of the most complicated problem in the kernel drivers development. For example, a target driver for Qlogic 2200/2300 cards, which has all necessary features, is only ~2000 lines of code long.
• Very low overhead, fine-grained locks and simplest commands processing path, which allow to reach maximum possible performance and scalability. Particularly, incoming requests can be processed in the caller's context or in one of the internal SCST's tasklets, therefore no extra context switches required.
• Provides advanced per-initiator device visibility management, which allows different initiators could see different set of devices with different access permissions. For instance, initiator A could see exported from target T devices Da and Db read-writable, and initiator B from the same target T could see devices Db read-only and Dc read-writable.
  
• Complete SMP support.
  
• Emulates necessary functionality of SCSI host adapter, because from a remote initiator's point of view SCST acts as a SCSI host with its own devices. Some of the emulated functions are the following:
  • Generation of necessary UNIT ATTENTIONs, their storage and delivery to all connected remote initiators (sessions).
  • RESERVE/RELEASE functionality.
  • CA/ACA conditions.
  • All types of RESETs and other task management functions.
  • REPORT LUNS command as well as SCSI address space management in order to have consistent address space on all remote initiators, since local SCSI devices could not know about each other to report via REPORT LUNS command. Additionally, SCST responds with error on all commands to non-existing devices and provides access control (not implemented yet), so different remote initiators could see different set of devices.
  • Other necessary functionality (task attributes, etc.) as specified in SAM-2, SPC-2, SAM-3, SPC-3 and other SCSI standards.
• Device handlers (i.e. plugins) architecture provides extra flexibility by allowing to make any additional requests processing, which is completely independent from target drivers and SCST core, like, for instance, device dependent exceptional conditions treatment, data caching or passing commands in the user space for device emulation. Example of such plugin is FILEIO device handler (see below) .
  
• Well documented.
  

Interoperability between SCST and local SCSI initiators (like sd, st) is the additional issue that SCST is going to address (it is not implemented yet). It is necessary, because local SCSI initiators can change the state of the device, for example RESERVE the device, or some of its parameters and that would be done behind SCST, which could lead to various problems. Thus, RESERVE/RELEASE commands, locally generated UNIT ATTENTIONs, etc. should be intercepted and processed as if local SCSI initiators act as remote SCSI initiators connected to SCST. This feature requires some the kernel modification. Since in the current version it is not implemented, SCST and target drivers are able to work with any 2.4 or 2.6 kernel version.

Interface between SCST and target drivers is based on work, done by University of New Hampshire Interoperability Labs (UNH IOL) for the UNH-iSCSI project, which is currently developed on SourceForge.net.

The latest stable version is 0.9.4. It supports SCSI disks (type 0), tapes (type 1), processors (type 3), CDROMs (type 5), MO disks (type 7), medium changers (type 8) and RAID controllers (type 0xC). Also it supports FILEIO and "performance" device handlers. FILEIO device handler allows to use files on file systems or block devices as virtual remotely available SCSI disks or CDROM's. "Performance" device handlers provide a way for direct performance measurements without overhead of actual data transferring from/to underlying SCSI device. Requires Linux kernel 2.6.14 or higher. Tested on i386 and x86_64 only, but should also work on any other supported by Linux platform. More detail information you could find in the project README file.

Starting from version 0.9.4 2.4 Linux kernels are not supported anymore, although there could be new SCST releases those kernels with very important bug fixes. The latest stable version of SCST with 2.4 kernels support is 0.9.3.1-24.

If you have any questions you can ask them on the SCST SF.net page either using forum, or scst-devel mailing list.

The latest development versions of SCST and its drivers are available directly from the project's SVN. You can access it using either web-based SVN repository viewer or using anonymous access:

svn co https://svn.sourceforge.net/svnroot/scst

More information about accessing SVN repository may be found here.

History of the pre-SVN SCST development is available in SCST CVS repository, which is accessible using web-based CVS repository viewer or anonymous CVS access.

Target driver for Qlogic 2200/2300 cards for 2.6 kernels

Target driver for Qlogic 2200/2300 cards

Target drivers for Adaptec 7xxx, LSI/MPT and Qlogic QLA12xx cards

Patches for UNH-iSCSI Target 1.5.03 and 1.6.00 to work over SCST