sg_luns (8) - Linux Manuals
sg_luns: send SCSI REPORT LUNS command or decode given LUN
NAMEsg_luns - send SCSI REPORT LUNS command or decode given LUN
SYNOPSISsg_luns [--decode] [--help] [--hex] [--linux] [--maxlen=LEN] [--quiet] [--raw] [--select=SR] [--verbose] [--version] DEVICE
In the first form shown in the SYNOPSIS this utility sends the SCSI REPORT LUNS command to the DEVICE and outputs the response. The response should be a list of LUNs ("a LUN inventory") for the I_T nexus associated with the DEVICE. Roughly speaking that is all LUNs that share the target device that the REPORT LUNS command is sent through. In the SPC-3 and SPC-4 SCSI standards support for the REPORT LUNS command is mandatory.
When the --test=ALUN option is given (the second form in the SYNOPSIS), then the ALUN value is decoded as outlined in SAM-3, SAM-4 and SAM-5 (revision 13, section 4.7) .
OPTIONSArguments to long options are mandatory for short options as well.
- -d, --decode
- decode LUNs into their component parts, as described in the LUN section of SAM-3, SAM-4 and SAM-5.
- -h, --help
- output the usage message then exit.
- -H, --hex
- [device mode] when given once this utility will output the SCSI response (i.e. the data-out buffer) to the REPORT LUNS command in ASCII hex then exit. When given twice it causes --decode to output component fields in hex rather than decimal.
- -H, --hex
- [test mode] when this option is given, then decoded component fields of ALUN are output in hex.
- -l, --linux
- this option is only available in Linux. After the T10 representation of each 64 bit LUN (in 16 hexadecimal digits), if this option is given then to the right, in square brackets, is the Linux LUN integer in decimal. If the --hex option is given twice (e.g. -HH) as well then the Linux LUN integer is output in hexadecimal.
- -m, --maxlen=LEN
- where LEN is the (maximum) response length in bytes. It is placed in the cdb's "allocation length" field. If not given (or LEN is zero) then 8192 is used. The maximum allowed value of LEN is 1048576.
- -q, --quiet
- output only the ASCII hex rendering of each report LUN, one per line. Without the --quiet option, there is header information printed before the LUN listing.
- -r, --raw
- output the SCSI response (i.e. the data-out buffer) in binary (to stdout).
- -s, --select=SR
this option sets the SELECT REPORT field (SR) in the SCSI REPORT
LUNS command. The default value is 0. For detailed information see the
REPORT LUNS command in SPC (most recent is SPC-4 revision 36e in section
6.33). To simplify, for the I_T nexus associated with the DEVICE, the
meanings of the SR values defined to date for SPC-4 are:
0 : all LUNs excluding well known logical units
1 : well known logical units
2 : all LUNs
Values between 0xf8 and 0xff (inclusive) are vendor specific (SPC-4 rev 36e), other values greater than 2 are reserved. This utility will accept any value between 0 and 255 (0xff) for SR .
- -t, --test=ALUN
- ALUN is assumed to be a hexadecimal number in ASCII hex or the letter 'L' followed by a decimal number (see below). The hexadecimal number can be up to 64 bits in size (i.e. 16 hexadecimal digits) and is padded to the right if less than 16 hexadecimal digits are given (e.g. --test=0122003a represents T10 LUN 0122003a00000000). ALUN may be prefixed by '0x' or '0X' (e.g. the previous example could have been --test=0x0122003a). ALUN may also be given with spaces or tabs between each byte (or other grouping) but then ALUN would need to be surrounded by single or double quotes. In the decimal number case (i.e. following a 'L') that number is assumed to be a Linux "word flipped" LUN which is converted into a T10 LUN representation and printed. In both cases the number is interpreted as a LUN and decoded as if the --decode option had been given. Also when ALUN is a hexadecimal number it can have a trailing 'L' in which case the corresponding Linux "word flipped" LUN value is output. The LUN is decoded in all cases.
- -v, --verbose
- increase the level of verbosity, (i.e. debug output).
- -V, --version
- print the version string and then exit.
NOTESThe SCSI REPORT LUNS command is important for Logical Unit (LU) discovery. After a target device is discovered (usually via some transport specific mechanism), a REPORT LUNS command should either be sent to LUN 0 (which is Peripheral device addressing method with bus_id=0 and target/lun=0) or to the REPORT LUNS well known LUN (i.e. 0xc101000000000000). SAM-5 requires that one of these responds with an inventory of LUNS that are contained in this target device.
In test mode, if the --hex option is given once then in the decoded output, some of the component fields are printed in hex with leading zeros. The leading zeros are to indicate the size of the component field. For example: in the Peripheral device addressing method (16 bits overall), the bus ID is 6 bits wide and the target/LUN field is 8 bits wide; so both are shown with two hex digits (e.g. bus_id=0x02, target=0x3a).
EXAMPLESTypically by the time user space programs get to run, SCSI LUs have been discovered. In Linux the lsscsi utility lists the LUs that are currently present. The LUN of a device (LU) is the fourth element in the tuple at the beginning of each line. Below we see a target (or "I_T Nexus": "6:0:0") has two LUNS: 1 and 49409. If 49409 is converted into T10 LUN format it is 0xc101000000000000 which is the REPORT LUNS well known LUN.
We could send a REPORT LUNS command to either /dev/sdb, /dev/sg1 or /dev/sg2 and get the same result. Below we use /dev/sg1 :
That is a bit noisy so cut down the clutter with --quiet:
Now decode that LUN into its component parts:
Would like to see how wide that component LUN field is:
So it is 8 bits wide (actually between 5 and 8 bits wide, inclusive). Now use --select=1 to find out if there are any well known LUNs:
So how many LUNs do we have all together (associated with the current I_T Nexus):
The following example uses the --linux option and is not available in other operating systems. The extra number in square brackets is the Linux version of T10 LUN shown at the start of the line.
Now we use the --test= option to decode LUNS input on the command line (rather than send a REPORT LUNS command and act on the response):
Peripheral device addressing: lun=0x4b
The next example is Linux specific as we try to find out what the Linux LUN 49409 translates to in the T10 world:
And the mapping between T10 and Linux LUN representations can be done the other way:
EXIT STATUSThe exit status of sg_luns is 0 when it is successful. Otherwise see the sg3_utils(8) man page.
AUTHORSWritten by Douglas Gilbert.
REPORTING BUGSReport bugs to <dgilbert at interlog dot com>.
COPYRIGHTCopyright © 2004-2013 Douglas Gilbert
This software is distributed under a FreeBSD license. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.