The Linux SCSI Generic (sg) Driver

1. The Linux SCSI Generic (sg) Driver

   1. News

   2. Introduction

   3. Background

   4. Features

   5. SG device driver downloads

   6. Utilities: sg3_utils and sg_utils

   7. Sg related pages

   8. External references

News

[2023-08-01] release of sg3_utils-1.48 . See sg3_utils page.

[2023-05-02] Updated scsi_debug page for the driver around lk 6.3

[2023-05-02] lsscsi pre-release 0.33, see lsscsi page

[2022-10-02] Start page on ChargerLab’s KM002C USB-C PD 3.1 tester

[2021-11-10] release of sg3_utils-1.47 . See sg3_utils page.

[2021-08-23] Updated sg version 4 driver. See the sg_v40 page, this driver is in development

[2021-05-05] lsscsi-0.32 released, see lsscsi page.

[2021-04-21] release of ddpt version 0.97 . See ddpt page.

[2021-04-21] release of sdparm version 1.12 . See sdparm page

[2021-04-08] This site (i.e. danny.cz ) now supports https, please adjust your browsers; caches may need to be cleared

[2021-02-03] lsscsi-0.32 beta, see lsscsi page.

[2020-03-19] release of smp_utils version 0.99 . See smp_utils page

[2020-02-20] lsscsi-0.31 released, see lsscsi page.

The author's various SCSI related packages are now mirrored on github.com . The primary repositories remain in subversion on the author's equipment. The sg3_utils and lsscsi packages are mirrored on Hannes Reinecke's site at github : https://github.com/hreinecke/sg3_utils in the master (default) branch. The author continues to track progress with a subversion revision number which is a sequential number currently at 891 for sg3_utils. The github sg3_utils package's ChangeLog file shows where the mirror is "at" and currently that is revision 841 dated 20200210, several commits behind the author's repository. The other sg3_utils git branches may be of interest to users and relate to Hannes' work at SUSE Labs. Most work in those other branches will be merged into the master branch in time. Also found at that location is the lsscsi mirror. Both packages may have later versions on this site (as tarballs in the Downloads section of the links given above). So it might be worthwhile to check both locations.

The author also has a github account at https://github.com/doug-gilbert which has mirrors for the sdparm, ddpt and smp_utils packages and alternative mirrors for sg3_utils and lsscsi. The sdparm and ddpt packages depend on a library from sg3_utils.

So, for example to build sdparm, clone the github sdparm and sg3_utils_lib repositories to the build machine; then from sg3_utils_lib clone, copy the include/ and lib/ directories with those names and contents over to the sdparm clone, under the main directory. So that main directory should end up at least containing these sub-directories: src/, doc/, lib/ and include/. The sdparm package should be then ready to build. IMO, contrary to much online propaganda, git is not better in all respects than subversion, especially for non-distributed development. In this case the author uses subversion's "externals" feature to include the library sub-tree of sg3_utils in the source trees of sdparm and ddpt. There is no exact parallel to this feature in git.

Packages will still have major releases done through this site. For example, that will be release 1.46 for sg3_utils . The release will have source tarballs and Debian and Redhat packages for Linux. There will also be a tarball of Windows executable(s) typically built with MinGW. Also there is an attempt to publicise the releases on related newsgroups (e.g. Note that the subversion revision number stops increasing in the ChangeLog once a release has been made, and a new paragraph will appear at the top of the ChangeLog when revisions are committed for the next release.

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

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, 3 and 4 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

See the sg3_utils page for more packages (e.g. 64 bit versions of Debian, Redhat and Windows binary packages) plus older versions.

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 disks 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 systemd and udev

• 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

• 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 that address.
Last updated: 1^st August 2023, 23:45 [UTC-4]