Generic SCSI Target Middle Level for Linux

The SCSI target mid-level subsystem for Linux (SCST) is a 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, SAS, Fibre Channel or 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.
  
• Device handlers, i.e. plugins, architecture provides extra flexibility by allowing various I/O modes in backstorage handling. For example, pass-through device handlers allows to use real SCSI hardware and vdisk device handler allows to use files as virtual disks.
• Provides advanced per-initiator device visibility management (LUN masking), which allows different initiators to 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.
  
• Emulates necessary functionality of SCSI host adapter, because from remote initiators point of view SCST acts as a SCSI host with its own devices. This is especially important in pass-through mode with one to many relationship, i.e. when multiple initiators can connect to the exported pass-through devices. You can find more deep elaboration why it is needed in this message in linux-scsi mailing list. Some of the emulated functions are the following:
  • Generation of necessary UNIT ATTENTIONs, their storage and delivery to all connected remote initiators.
  • RESERVE/RELEASE functionality.
  • CA/ACA conditions (not implemented yet).
  • 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, 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.
• Multithreaded design and complete SMP support, so all your processors will participate in the commands processing.
  
• Well documented.
  

Interoperability between SCST and local SCSI initiators (i.e. sd, st, etc.) 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 could be done behind SCST, which could lead to various problems, including data corruption. 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.

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 was developed on SourceForge.net.

SCST supports the following I/O modes:

• Pass-through mode with one to many relationship, i.e. when multiple initiators can connect to the exported pass-through devices, for virtually all SCSI devices types: 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)
• FILEIO mode, which allows to use files on file systems or block devices as virtual remotely available SCSI disks or CDROMs with benefits of the Linux page cache
  
• BLOCKIO mode, which performs direct block IO with a block device, bypassing page-cache for all operations. This mode works ideally with high-end storage HBAs and for applications that either do not need caching between application and disk or need the large block throughput.
• User space mode using scst_user device handler, which allows to implement in the user space virtual SCSI devices in the SCST environment
  
• "Performance" device handlers, which provide in pseudo pass-through mode a way for direct performance measurements without overhead of actual data transferring from/to underlying SCSI device
  

The latest stable version is 0.9.5. Requires Linux kernel 2.6.14 or higher. Tested mostly on i386 and x86_64, but should also work on any other supported by Linux platform. More detail information you could find in the SCST README file.

If you have any questions you can ask them via scst-devel mailing list. See main SCST SF.net page  for more info about SCST mailing lists.

The latest development version of SCST and target drivers is available directly from the project's SVN. The SCST release policy is to make stable releases once or twice a year, with the only exception if the current stable version contains a critical bug. But every commit in the SVN is carefully tested, so you can consider them as mini releases. Therefore on practice the development version in the SVN is usually more stable, than the "stable", i.e. released, one. You can access it using either web-based SVN repository viewer or using anonymous access:

svn co https://scst.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.

Main SCST project page on SF.net
Download released versions
Change Log

Index