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SG_MAP26

Section: SG3_UTILS (8)
Updated: April 2023
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NAME

sg_map26 - map SCSI generic (sg) device to corresponding device names

SYNOPSIS

sg_map26 [--dev_dir=DIR] [--given_is=0|1] [--help] [--result=0|1|2|3] [--symlink] [--verbose] [--version] DEVICE

DESCRIPTION

Maps a special file (block or char) associated with a SCSI device to the corresponding SCSI generic (sg) device, or vice versa. Can also be given a sysfs file, for example '/sys/block/sda' or '/sys/block/sda/dev'.

Rather than map to or from a sg device, the sysfs file name matching a given device special file (or vice versa) can be requested. This is done with '--result=2' and '--result=3'. This feature works on ATA devices (e.g. 'dev/hdc') as well as SCSI devices.

In this utility, "mapped" refers to finding the relationship between a SCSI generic (sg) node and the higher level SCSI device name; or vice versa. For example '/dev/sg0' may "map" to '/dev/sda'. Mappings may not exist, if a relevant module is not loaded, for example. Also there are SCSI devices that can only be accessed via a sg node (e.g. SAF-TE and some SES devices).

In this utility, "matching" refers to different representations of the same device accessed via the same driver. For example, '/dev/hdc' and '/sys/block/hdc' usually refer to the same device and thus would be considered matching. A related example is that '/dev/cdrom' and '/dev/hdc' are also considered matching if '/dev/cdrom' is a symlink to '/dev/hdc'.

Note that this utility requires root permissions to access the storage device nodes in the /dev directory. The lsscsi utility may be used instead in many cases, and lsscsi does not require root permission as it "data-mines" the sysfs pseudo file system for the information it needs.

For notes on bsg and nvme device nodes see the section on BSG and NVME DEVICES below.

OPTIONS

Arguments to long options are mandatory for short options as well.

-d, --dev_dir=DIR: where DIR is the directory to search for resultant device special files in (or symlinks to same). Only active when '--result=0' (the default) or '--result=2'. If this option is not given and DEVICE is a device special file then the directory part of DEVICE is assumed. If this option is not given and DEVICE is a sysfs name, then if necessary '/dev' is assumed as the directory.
-g, --given_is=0 | 1: specifies the DEVICE is either a device special file (when the argument is 0), or a sysfs 'dev' file (when the argument is 1). The parent directory of a sysfs 'dev' file is also accepted (e.g. either '/sys/block/sda/dev' or '/sys/block/sda' are accepted). Usually there is no need to give this option since this utility first checks for special files (or symlinks to special files) and if not, assumes it has been given a sysfs 'dev' file (or its parent). Generates an error if given and disagrees with variety of DEVICE.
-h, --help: output the usage message then exit.
-r, --result=0 | 1 | 2 | 3: specifies what variety of file (or files) that this utility tries to find. The default is a "mapped" device special file, when the argument is 0. When the argument is 1, this utility tries to find the "mapped" sysfs node name. When the argument is 2, this utility tries to find the "matching" device special file. When the argument is 3, this utility tries to find the "matching" sysfs node name.
-s, --symlink: when a device special file is being sought (i.e. when '--result=0' (the default) or '--result=2') then also look for symlinks to that device special file in the same directory.
-v, --verbose: increase the level of verbosity, (i.e. debug output).
-V, --version: print the version string and then exit.

BSG and NVME DEVICES

The bsg driver (Block Scsi Generic) presents alternate Unix character devices of the form: /dev/bsg/<h:c:t:l>. <h:c:t:l> is a 4 element tuple where 'h' is the host number, 'c' is the controller number, 't' is the target identifier and 'l' is the LUN (Logical Unit Number). Other than the bsg device names, each device, once open()-ed, operates in a similar fashion to a sg device: a generic interface at the SCSI command level.

If a bsg device is given to this utility, it will first try to map it to a SCSI block device. This will either be of the form /dev/sd<letter_s> for a disk or /dev/sr<number> for a BD/DVD or CD drive. If no match is found, then a check is made for a SCSI tape drive with a name of the form /dev/st<number>. If that fails, the mapping to the corresponding sg device is made. If the --result=2 option is given, the block device and tape davice checks are bypassed and the mapping to the corresponding sg device is made.

Note that this utility makes no mapping to a bsg device. That is pretty easy to establish using the lsscsi utility as the 4 element <h:c:t:l> tuple is shown at the beginning of each line (by default) and it uniquely identifies each SCSI device. Each line will also show the primary device name and, if the --generic option is given, the sg device node.

Currently NVMe device nodes come in several varieties, block devices of the form: /dev/nvme<c>n<n>[p<pn>] where <c> is the controller number (starting at 0), <n> is the namespace identifier (starting at 1), and <pn> is the partition number (starting at 1). There are two forms of NVMe character device nodes: one of the form /dev/nvme<c> and the other of the form /dev/ng<c>n<n> where "ng" is newer and termed as 'nvme-generic' in the /proc/devices output. This utility identifies but does not map NVMe devices. Their naming is more consistent so a utility like this is less needed. Udev might be used to remap the kernel's naming scheme for NVMe, removing its inherent naming consistency.

NOTES

This utility is designed for the Linux 2.6 (and later) kernel series. It uses special file major and minor numbers (and whether the special is block or character) together with sysfs to do its mapping or matching. In the absence of any other information, device special files are assumed to be in the '/dev' directory while sysfs is assumed to be mounted at '/sys'. Device names in sysfs are predictable, given the corresponding major and minor number of the device. However, due to udev rules, the name of device special files can be anything the user desires (e.g. '/dev/sda' could be named '/dev/my_boot_disk'). When trying to find a resultant device special file, this utility uses the major and minor numbers (and whether a block or char device is sought) to search the device directory.

This utility only shows one relationship at a time. To get an overview of all SCSI devices, with special file names and optionally the "mapped" sg device name, see the lsscsi utility.

Even though lsscsi is a functional replacement for this utility, it has been reported that this utility runs faster in systems that have a lot of disks connected. Theoretically a Linux system may have up to 16383 SCSI disks connected to it.

EXAMPLES

Assume sg2 maps to sdb while dvd, cdrom and hdc are all matching.

# sg_map26 /dev/sg2
/dev/sdb

# sg_map26 /dev/sdb
/dev/sg2

# sg_map26 --result=0 /dev/sdb
/dev/sg2

# sg_map26 --result=3 /dev/sdb
/sys/block/sda

# sg_map26 --result=1 /dev/sdb
/sys/class/scsi_generic/sg0

Now look at '/dev/hdc' and friends

# sg_map26 /dev/hdc
<error: a hd device does not map to a sg device>

# sg_map26 --result=3 /dev/hdc
/sys/block/hdc

# sg_map26 --result=2 /dev/hdc
/dev/hdc

  # sg_map26 --result=2 --symlink /dev/hdc
  /dev/cdrom
  /dev/dvd
  /dev/hdc

  # sg_map26 --result=2 --symlink /sys/block/hdc
  /dev/cdrom
  /dev/dvd
  /dev/hdc

EXIT STATUS

The exit status of sg_map26 is 0 when it is successful. Otherwise see the sg3_utils(8) man page.

AUTHORS

Written by Douglas Gilbert.

REPORTING BUGS

Report bugs to <dgilbert at interlog dot com>.

COPYRIGHT

This document was created by man2html, using the manual pages.
Time: 04:00:44 GMT, August 02, 2023