TapeSAN Setup for Systems

Background information about methods for identifying tapes and media changers 

On older systems, most tape drives were connected to the system through SCSI. In those cases, the system needed only to scan the SCSI bus to identify all the peripherals that were attached to it. However, if drives were removed or added after the system was booted, the system was not aware of the changes. A reboot was required before the configurations took effect. This method of direct connect SCSI tape device did not allow for hot swapping or dynamically changing tape configurations after the system was running.

This Fibre Channel fabric TapeSAN backup solution enables hot swapping and dynamically changing tape configurations after the system is running. A Fibre Channel tape drive or library can now be connected and disconnected from the Fibre Channel switch, and the switches rescans the ports to determine whether devices were added or removed. The system detects these configuration changes in real time. It disables access to drives that disappeared, and it exports new drives for the user.

Device naming convention

After a tape drive is discovered by the system, the drive is allocated a name using the format stn, where n is a number assigned to the drive based on when it was discovered. Tape libraries (also known as media changers mc) follow a similar naming convention. The following are some limitations with earlier naming conventions:

  • Data ONTAP assigns device addresses on a first-come first-served basis.
  • Assigned stn addresses do not move when the device is moved.
  • Addresses might not be reassigned in the same order after reboot

Unlike direct attached SCSI devices, the order of device discovery on a SAN is not fixed. New tape drives can be easily added or removed from the system. Names assigned to one device can be assigned to a different device after a reboot. This dynamic naming scheme is problematic with tape drives and libraries on a SAN. To solve this naming scheme problem, the tape aliasing method was developed.

Tape aliasing

Tape aliasing provides a correspondence between the logical names of a backup device, for example (st0, mc1), and a name permanently assigned to a port or to a tape device. The following are three naming schemes used to assign permanent names to logical names of tape devices:

Logical names

A logical name is the software assigned name given to the tape device by the system. The format for tape drive logical name is stn, where n is an integer greater than or equal to 0. The value of n is assigned to the drive based on when it was discovered. Tape libraries (also known as media changers mc) follow a similar naming convention.

Example: Logical name - dump 0f /dev/nrst0a /vol/vol0

Electrical or Physical path names (PPN)

A PPN is the numerical address sequence that Data ONTAP assigns to a tape drive and library based on which SCSI-2/3 adapter or switch they are connected to on the system. A PPN is also known as an electrical name.

Note: Different systems can have different PPNs for the same tape device.

Example: Electrical name - dump 0f /dev/nr.MY_SWITCH:5.6.a /vol/vol0

World wide names (WWN)

A WWN is assigned to tape devices at the time of manufacture. WWN applies only to Fibre Channel devices and not to SCSI devices.

Note: Accessing a tape device through the WWN enables multiple systems to track the same device. Also, using WWNs eliminates confusion in naming conventions sometimes caused by using PPNs.

Example: World wide name - dump 0f /dev/nr.WWN[2:000:00e08b:01523e].a /vol/vol0

Mapping tape aliases

The logical name assigned by the system can be mapped to the device PPN or WWN. In this event, assigned stn or mcn names have direct association with PPNs and WWNs.

Example: Logical name st0 can be mapped to PPN MY_SWITCH:5.6.a /vol/vol0
OR
Example: Logical name st0 can be mapped to WWN WWN
2:000:00e08b:01523e].a /vol/vol0

New backup related system commands supported in Data ONTAP 6.1

  • Command: storage show tape - Provides the following information:
    • Tape drive PPN
    • Tape drive make and model
    • Serial number of the tape drive
    • Tape drive Fibre Channel WWN
    • Tape drive alias name
  • Command: storage show mc - Provides the following information:
    • Media changer/library PPN
    • Media changer/library make and model
    • Media changer/library serial number
    • Media changer/library Fibre Channel WWN
    • Media changer/library alias name
  • Command: storage adapter disable adapter # - Provides the following information:
    • Disables selected Host Bus Adapter
  • Command: storage alias - Provides the following information:
    • Displays device alias
  • Command: storage unalias - Provides the following information:
    • Removes device alias