When a library is "autoconfigured", the device path (scsi path on Windows or device file on *UX) will automatically be configured along with the serial number and drive index for each drive in the library and everything will work fine. However, in some cases it is still important to understand what this "Drive index" really means.

The "Drive index" configured with the tape device in DP can be seen as the link between the tape device and the library device or robotics. When a media agent is started for a specific logical device, one of the first things that will happen is start a uma to load a tape into the device Here the drive index will be used to let the library (robotics) know the destination of the move operation. If the drive index is "1" then the tape will be loaded into the frist drive in the library. Note that this move command has no relation with the configured device path. Whatever path is configured for this logical device (wrong or right), the tape will get loaded to the device with the configured drive index. After the tape has been loaded, the configured device path will be used to access the tape. If the drive index and the device path would not really point to the same device in the library then the media agent will obviously fail to access the tape.

The serial number of the device can be used as a link between the path and the index. A "devbra" output will not only show serial numbers for each tape device, but it will also list the serial numbers for each device in the library as part of the library device or robotics.

C:\Users\Administrator>devbra -dev

Tape    HP:Ultrium 6-SCSI Path: "scsi3:0:2:0C" SN: "CZ15200KYZ"
        Description: HP LTO6 Drive
        Revision: ED61 Device type: lto [13] Flags: 0x0001

Tape    HP:Ultrium 6-SCSI Path: "scsi3:0:1:0C" SN: "CZ15200KYY"
        Description: HP LTO6 Drive
        Revision: ED61 Device type: lto [13] Flags: 0x0001

Exch    HP:ESL E-Series Path: "scsi3:0:0:0" SN: "CZ15200L2W"
        Description: HP StorageWorks ESL E-Series
        Revision: EL51 Flags: 0x0006 Slots: 24 Drives: 2
        Drive(s) SN:
                "CZ15200KYY"
                "CZ15200KYZ"

Note the "Drive(s) SN" list of serial numbers, which is an indication of the drive index for each drive (the list is sorted that way). Also note that it would be wrong to trust the order of the tape devices itself as they are ordered the other way around in the example above. So, "CZ15200KYY" is D1, the first drive in the library and "CZ15200KYZ" is D2, the second drive in the library. The corresponding device paths are "scsi3:0:1:0C" and "scsi3:0:2:0C" as can be seen by matching the serial numbers. And this is exactly what "sanconf" is doing for us to produce an output like this:

C:\Users\Administrator>sanconf -list

<<<Lines cut>>>

========================== SUMMARY REPORT ==========================

The following devices CAN be configured:

LIBRARY="HP:ESL E-Series" serial number="CZ15200L2W"
               HOST: system.domain@system.domain DEVICE: scsi3:0:0:0
        DRIVE: index=1; name="HP:Ultrium 6-SCSI"; serial number="CZ15200KYY"
               HOST: system.domain@system.domain DEVICE: scsi3:0:1:0C
        DRIVE: index=2; name="HP:Ultrium 6-SCSI"; serial number="CZ15200KYZ"
               HOST: system.domain@system.domain DEVICE: scsi3:0:2:0C
====================================================================

And in the GUI, when clicking the "Arrow-Down" to detect the device path, a devbra will actually be started in the background to discover the relation between devices in the library and device paths (based on serial number matching).

You may be interested to some of my other Support Tips too: Support Tips listed per category.