Can not find tape drive ID using using 'mtx -f /dev/sg4 status' command

Environment

• Red Hat Enterprise Linux 5, 6, 7
• Tape devices, medium changer

Issue

• Can not find tape drive ID using using mtx -f /dev/sg4 status command.

• mtx -f /dev/sg4 status command shows error Illegal Request

    $ mtx -f /dev/sg4 status
    mtx: Request Sense: Long Report=yes
    mtx: Request Sense: Valid Residual=no
    mtx: Request Sense: Error Code=70 (Current)
    mtx: Request Sense: Sense Key=Illegal Request
    mtx: Request Sense: FileMark=no
    mtx: Request Sense: EOM=no
    mtx: Request Sense: ILI=no
    mtx: Request Sense: Additional Sense Code = 20
    mtx: Request Sense: Additional Sense Qualifier = 00
    mtx: Request Sense: BPV=no
    mtx: Request Sense: Error in CDB=no
    mtx: Request Sense: SKSV=no
    READ ELEMENT STATUS Command Failed
  

Resolution

• Please use the mtx -f /dev/sgX <arg.> command with tape changer, autoloader, tape library or medium changer only. It would not work as expected for tape devices.

• To check the status of tape device, you could use following command instead of mtx -f /dev/sgX <arg.>:

  o First verify if the st module is successfully loaded:

    $ lsmod |grep -i st
  

  o If above module is not loaded, then please load the same manually using following command:

    $ modprobe st
  

  o Execute following command to check the status of tape device.

    $ mt -f /dev/st0 status
  

Root Cause

• The mtx command would not work as expected for the tape devices, since it is suppose to be used with tape changer, autoloader or tape library devices only. Below is the snip from man page of mtx command (man 1 mtx):

    NAME
           mtx - control SCSI media changer devices
    
    SYNOPSIS
           mtx [-f <scsi-generic-device>] [nobarcode] [invert] [noattach] command [ command ... ]
    
    DESCRIPTION
           The  mtx  command controls single or multi-drive SCSI media changers such as tape changers, autoloaders, tape libraries, or optical media jukeboxes.  It can also be used with media changers that use the ’ATTACHED’ API, presuming that they properly report the MChanger bit as required by the SCSI T-10 SMC specification.
    
    OPTIONS
           The first argument, given following -f , is the SCSI generic device corresponding to your media changer. 
    [...]
  

• The information in /proc/scsi/scsi and lsscsi command output shows that /dev/sg4 device corresponds to HP Ultrium 6-SCSI tape device and not the tape changer, autoloader or tape library, due to which mtx -f /dev/sg4 status command showed an error Sense Key=Illegal Request.

    $ lsscsi --generic
    [6:0:0:0]    disk    FUJITSU  ETERNUS_DXL      1032  /dev/sda   /dev/sg0 
    [6:0:0:1]    disk    FUJITSU  ETERNUS_DXL      1032  /dev/sdb   /dev/sg1 
    [7:0:1:0]    disk    FUJITSU  ETERNUS_DXL      1032  /dev/sdc   /dev/sg2 
    [7:0:1:1]    disk    FUJITSU  ETERNUS_DXL      1032  /dev/sdd   /dev/sg3 
    [7:0:2:0]    tape    HP       Ultrium 6-SCSI   251B  /dev/st0   /dev/sg4 	<---
    
    Host: scsi7 Channel: 00 Id: 02 Lun: 00
      Vendor: HP       Model: Ultrium 6-SCSI   Rev: 251B
      Type:   Sequential-Access                ANSI  SCSI revision: 06
  

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