The Linux SCSI Generic (sg) Driver

The Linux SCSI Generic (sg) Driver

News

[2014-07-12] beta version sg3_utils-1.40b2, sg_tst_async, see ChangeLog in tarball
[2014-07-10] proposed scsi_debug driver version 1.84 for lk 3.17
[2014-06-12] release sg3_utils version 1.39, see the sg3_utils page
[2014-05-26] release smp_utils 0.98 (for SAS expander management), see smp_utils page
[2014-04-07] release ddpt version 0.94, see ddpt page
[2014-01-27] beta lsscsi version lsscsi-0.28b1, see ChangeLog in tarball
[2013-06-09] release sdparm version 1.08 , see the sdparm page
[2013-05-08] release lsscsi version 0.27 , see lsscsi page
[2010-09-13] add device naming page
[2008-10-01] sg pages find a new home at http://sg.danny.cz/sg/
[2007-10-29] update tools page: brief summary of various storage/scsi tools

Introduction

The Linux sg driver is a upper level SCSI subsystem device driver that is used primarily to handle devices _not_ covered by the other upper level drivers: sd (disks), st (tapes) and sr (CDROMs and DVDs). The sg driver is used for enclosure management, cd writers, applications that read cd audio digitally and scanners. Sg can also be used for less usual tasks performed on disks, tapes and cdroms. Sg is a character device driver which, in some contexts, gives it advantages over block device drivers such as sd and sr. The interface of sg is at the level of SCSI command requests and their associated responses.

The term SCSI has several meaning depending on the context. This leads to confusion. One practical way of defining it today is everything that the T10 INCITS committee controls, see www.t10.org . Probably the most succinct overview is this standards architecture page . For practical purposes a "SCSI device" in Linux is any device that uses the Linux SCSI subsystem and this often includes SATA disks.

From about Linux kernel 2.6.24, there is an alternate SCSI pass-through driver called "bsg" (block SCSI generic driver). The bsg driver has device names of the form /dev/bsg/0:1:2:3 and supports the SG_IO ioctl with the sg version 3 interface. The bsg driver also supports the sg version 4 interface which at this time the sg driver does not. Amongst other improvements the sg version 4 interface supports SCSI bidirectional commands. All recent "sg" user space packages (i.e. sg3_utils, sdparm, ddpt and smp_utils) work equally well on both sg and bsg device names.

Background

The original driver was written by Lawrence Foard in 1992 and remained unchanged for several years. In August 1998 Heiko Eissfeldt and Joerg Schilling started working on enhancements to this driver. Soon after, the author became involved and these efforts culminated in a new sg driver being placed in Linux kernel 2.2.6 which was released on 16th April 1999. It contains the first major upgrade to the SCSI generic packet device driver ("sg") since 1992. This new driver has a super-set of the original interface and the semantics of the implementation are very similar. Hence it offers a high degree of backward compatibility with the original driver.

The major reason for introducing a new sg driver into the 2.2 series of kernels was the problem that the original device driver was having finding memory. This driver improves the situation by using scatter gather, memory above the 16 MBytes level and memory from the scsi dma pool as appropriate. Other drivers were affected by these memory problems (especially those associated with ISA hardware). In kernel 2.2.10 H.J. Lu introduced a new kernel memory allocator that alleviated many of these memory problems.

On 4th January 2001 the Linux 2.4.0 kernel was introduced and it contained the "version 3" sg driver that is described below.

On 17th December 2003 the Linux 2.6.0 kernel was introduced and it also contained the "version 3" sg driver that is described below. The SG_IO ioctl was implemented in the block layer and in several other "char" SCSI drivers (e.g. the st driver for tapes). Hence many programs can use primary device nodes (e.g. /dev/hdd for an ATAPI cd/dvd writer) to send SCSI command via the SG_IO ioctl. The sg driver still permits a clean pass through interface to all devices that use the Linux SCSI subsystem.

Features

The following enhancements have been added (in lk 2.2.6): scatter gather, command queueing, per file descriptor sequencing (was per device) and asynchronous notification. Scatter gather allows large buffers (previously limited to 128 KB on i386) to be used. Scatter gather is also a lot more "kernel friendly". The original driver used a single large buffer which made it impossible to run 2 or more sg-based applications at the same time. With the new driver a buffer is reserved for each file descriptor guaranteeing that at least that buffer size will be available for each request on the file descriptor. A user may request a larger buffer size on any particular request but runs the (usually remote) risk of an out of memory (ENOMEM) error.

A "version 3" sg driver was introduced in Linux kernel 2.4.0 . It adds a new interface that allows more control over SCSI commands and returns more information about their performance. This driver is present in Linux kernel 2.4.0 . A separate version with reduced capabilities is available for the 2.2 series kernels. Features include: a simplified SG_IO ioctl, larger sense buffer, residual DMA count, 16 byte (or longer) commands, direct IO support, command duration timing and a "proc_fs" interface. Naturally it is backward compatible with applications based on the sg interface in the lk 2.2 series and earlier.

In the lk 2.6 series the SG_IO ioctl has been replicated (with a slightly reduced feature set) in the block subsystem. This allows the SG_IO ioctl to be used on block devices such as /dev/sda and /dev/scd0. In lk 2.6.6 the SG_IO ioctl became available in the st upper level SCSI subsystem driver (for tapes). See the sg_io page.

SG device driver downloads

The following table summarizes the different versions of the sg device driver that are available. If you wish to use one of these tarballs then untar it in /usr/src/linux (or wherever the top of your kernel tree is). As a precaution you may wish to copy the files include/scsi/sg.h and drivers/scsi/sg.c to other names. This will facilitate reversing the patch if required. For information about the differences between versions see the history section at the top of the include/scsi/sg.h file.

**Table 1. sg device drivers**
Sg driver version+tarball	Notes
sg version 3.0.19	sg version 3 backported to lk 2.2
sg version 1.5.01	sg version 1 (original) ported to lk 2.4

sg3117_lk240	documentation and notes
sg_lk2422	bug fix
sg_lk2423	bug fix, found in lk 2.4.23->26
sg_lk2428	version found in lk 2.4.28
sg_lk2429	version found in lk 2.4.29,30,31,32

sg_lk2611	driver found in 2.6.11
sg_lk2613	driver found in 2.6.13
sg_lk2616	driver found in 2.6.16
sg_lk2618	driver found in 2.6.18
sg_lk2618scat	proposed additions including scatter_elem_sz attribute

For the Linux kernel 2.2 series, the current production version of the sg driver is 2.1.39 . All sg drivers for the 2.2 series of kernels (including the original) are interchangeable (but see final section on this page about Red Hat 6.2 and Mandrake 7.1). So, for example, sg22orig.tgz can be put in lk 2.2.17 while, on the other hand, sg2140.tgz can be put in lk 2.2.0 .

For the Linux kernel 2.4 series, the current production version of sg is 3.1.25 (in lk 2.4.23->32).

For the Linux kernel 2.6 series, the current production version of sg is 3.5.34 .

Older versions can be obtained from this directory. A sub directory called "original" contains the original driver (i.e. prior to linux kernel 2.2.6) and its HOWTO document.

Utilities: sg3_utils and sg_utils

These packages contain multiple utilities, all with command line interfaces. They send one one more SCSI commands to the nominated device and output the response including error reports if the command failed. The name of the SCSI command executed is typically reflected in the name of the utility; for example: "sg_inq" sends a SCSI INQUIRY command and decodes its response. These utilities were originally written for the Linux operating system. In sg3_utils version 1.19 a subset of the utilities were ported to FreeBSD and Tru64; in version 1.22 a similar subset of the utilities were ported to Windows; and in version 1.24 a Solaris port was added.

The most recent package is called sg3_utils and it uses the sg "version 3" interface (sometimes known as the SG_IO interface) in linux. This interface is in lk 2.4.0 and later kernels, including the lk 2.6 series. All new features are being added into sg3_utils (i.e. it is still being actively developed). Many of the utilities in sg3_utils (versions >= 1.02) can be used directly on block devices (e.g. sg_inq /dev/sda) in the lk 2.6 series. The sg_dd utility is at a slightly higher level issuing SCSI READ and WRITE commands to copy data, mimicking the Unix dd command. See the sg3_utils package page for more details.

The older (and original) package is called sg_utils and it uses the sg version 2 interface found linux kernel version 2.2.6 and onward. Over half of those utilities will also work on the original sg device driver (at least back to the lk 2.0 series). If required, "make common" will produce the subset that will work on the whole lk 2.x series. Since the sg "version 3" device driver still maintains the original interface, the sg_utils package will continue to work in the lk 2.4 and lk 2.6 series . This package is now in maintenance mode, only bugs will be fixed. See this page for more details.

The latest versions, packaged in tarballs, source, binary rpms and debian packages are given in the following table. They contain README and CHANGELOG files and man pages. The tarball contains a "spec" file for building rpms and a debian directory containing build information. Here is the most recently released sg3_utils CHANGELOG .

**Table 2. sg3_utils/sg_utils tarballs and packages**
	tarballs	rpm source rpms **	i386 rpm binaries **	debian packages
sg3_utils check News section above for beta	sg3_utils-1.39.tgz sg3_utils-1.39.tar.xz	sg3_utils-1.39-1.src.rpm	sg3_utils-1.39-1.i386.rpm sg3_utils-libs-1.39-1.i386.rpm	sg3-utils_1.39-0.1_i386.deb libsgutils2-2_1.39-0.1_i386.deb
sg_utils	sg_utils-1.02.tgz	sg_utils-1.02-1.src.rpm	sg_utils-1.02-1.i386.rpm

See the sg3_utils page for older versions and recent betas.

** Some browsers (e.g. firefox) may interpret a file with an extension of "rpm" as an audio file. In which case press the right button over the link and select "Save link as ..." to download the file.

Sg related pages

Following are some links to related pages on this site:

Full sg version 3 documentation for lk 2.4 series is now available. It is in DocBook format (html rendering): sg_v3_ho.html (ps, pdf and text renderings also in same directory). This document is also available as LDP's SCSI-Generic-HOWTO Here is documentation specific to the sg_io ioctl, especially in the lk 2.6 series.
Older Sg version 3 documentation for lk 2.4 series (now superseded by the DocBook version above)
lsscsi command for the lk 2.6 series. Lists SCSI devices or SCSI hosts.
sdparm utility to view and change mode pages, view VPD pages and send commands (for lk 2.6 and 2.4 series)
ddpt utility which is dd-like which is able to access SCSI devices at the SCSI command level
sg3_utils package of utilities that send SCSI commands
smp_utils package of utilities for Serial Attached SCSI (SAS) SMP functions
the tools page contains information about many scsi/storage packages from various sources
Some notes about the "version 3" sg driver found in the lk 2.4 series (including mmap notes).
scsi_debug LLD (adapter) driver for simulating lots of disk and doing tests in the lk 2.6 and lk 3 series. Here is the scsi_debug driver for the lk 2.4 series.
scsi_ses LLD (adapter) driver for simulating an enclosure services (SES) device.
Timings for direct and indirect IO (as used by sg); these are old and should be updated
Some fast copy options for disks (including raw devices)
devfs and SCSI in lk 2.3.46 and the lk 2.4 series. Devfs has been replaced by udev and hotplug in the lk 2.6 series
description of the Linux SCSI subsystem in the lk 2.4 series. There is a single page html rendering: SCSI-2.4-HOWTO.html plus ps, pdf and text renderings in the same directory. This document has become the LDP's SCSI-2.4-HOWTO . The version on this site may be more recent.
linux 2.6 series SCSI mid to lower level API document can be found in the Documentation/scsi/scsi_mid_low_api.txt file in the kernel source tree.
smartmontools for SCSI article .
Abridged and longer documentation for sg driver in lk 2.2 series (describes sg version 2.1.39)
A FAQ including current project status, bug fixes and other useful patches

External references

Here are some useful links to related information held at other sites:

www.t10.org is a very important site for SCSI related information. It contains SCSI 2 and 3 draft standards.
every wonder what all those acronyms mean and how they relate, see this diagram at t10.org .
www.scsita.org is the SCSI Trade Association. There is an interesting tutorial on Serial Attached SCSI (SAS) there.
www.andante.org/scsi.html is Eric Youngdale's site. Eric is primarily responsible for the Linux SCSI architecture and its mid-level implementation in the 2.2 and 2.4 series of kernels
Kurt Garloff has several pages with Linux SCSI themes (including the scsidev, scsiinfo and rescan-scsi-bus programs).
The Scsi Command Utility (scu) is quite comprehensive and is ported to several OSes including Linux. Its home page can be found here .
The newsgroup linux-scsi@vger.kernel.org is the appropriate place to discuss Linux SCSI related issues.
The MAINTAINERS source file in the top level of the linux kernel source tree contains up to date information about who maintains what and where information can be found. Links to SCSI adapter information are also in that file. SCSI documentation within the kernel source has been moved to the Documentation/scsi directory in the lk 2.6 series.

Douglas Gilbert can be emailed at this address.
Last updated: 12th July 2014, 20:30 [UTC-4]