Multipath I/O access with IBM tape devices

Multipath I/O is a technique that uses different paths to access the same physical device, for example through multiple host bus adapters (HBA) or switches. The use of the multipath technique helps to ensure that a single point of failure does not occur.

The IBM tape device driver provides multipathing support so that if one path fails, the server can use a different path to access data on a storage device. The failure and transition to a different path are undetected by the running server or by a storage agent. The IBM® tape device driver also uses multipath I/O to provide dynamic load balancing for enhanced I/O performance.

To provide redundant paths for IBM tape devices, connect each device to two or more ports on a multiport Fibre Channel or SAS Host Bus Adapter (if it is available on your operating system), or to different single Fibre Channel Host Bus Adapters. If multipath I/O is enabled and a permanent error occurs on one path, such as a malfunctioning HBA or cable, device drivers provide automatic path failover to an alternate path.

After multipath I/O is enabled, the IBM tape device driver detects all paths for a device on the host system. One path is designated as the primary path. The rest of the paths are alternate paths. The maximum number of alternate paths for a device is 16. For each path, the IBM tape device driver creates a special file with a unique name. A path must exist on the system before the driver can create a special file for the path. If a path does not exist, the driver does not create a special file. When you use the DEFINE PATH command to specify the path to a destination, specify the file that is associated with the primary path as the value of the DEVICE parameter.

On AIX®, multipath I/O is not enabled automatically when the IBM tape device driver is installed. You must configure it for each logical device after installation. Multipath I/O remains enabled until the device is deleted or the support is unconfigured. For configuration instructions, see the IBM Tape Device Drivers Installation and User's Guide.

To obtain the names of special files, use the ls -l command, for example, ls -l /dev/rmt*. Primary paths and alternate paths are identified by PRI and ALT, as seen in the following example:

rmt0 Available 20-60-01-PRI IBM 3590 Tape Drive and Medium Changer (FCP)
rmt1 Available 30-68-01-ALT IBM 3590 Tape Drive and Medium Changer (FCP)

In this example, the following paths are associated with the IBM 3590 tape drive:

20-60-01-PRI
30-68-01-ALT

The name of the special file that is associated with the primary path is /dev/rmt0. Specify /dev/rmt0 as the value of the DEVICE parameter in the DEFINE PATH command.

To display path-related details about a particular tape drive, you can also use the itdt -f /dev/rmtx path command, where x is the number of the configured tape drive. To display path-related details about a particular medium changer, use the itdt -f /dev/smcy path command, where y is the number of the configured medium changer.

On Linux, multipath I/O for medium changers and tape drives is not enabled automatically when the device driver is installed. For instructions about configuring multipath I/O, see the IBM Tape Device Drivers Installation and User's Guide.

When multipath I/O is enabled for a logical device, it remains enabled until the device is deleted or the support is unconfigured.

To display the names of special file for IBM tape drives and medium changers, use the ls -l /dev/IBMx, where x is the index number of the device. You can also enter the cat /proc/scsi/IBMtape command for tape drives. As shown in the IBMtape file, primary paths and alternate paths are identified as Primary or Alternate:

 Number     Model       SN              HBA            FO Path
  0         03592       IBM1234567      qla2xxx        Primary
  1         03592       IBM1234567      qla2xxx        Alternate

The name of the special file that is associated with the primary path for this tape drive is /dev/IBMtape0. Specify /dev/IBMTape0 as the value of the DEVICE parameter in the DEFINE PATH command for this device.

To obtain the names of the special files that are associated with the primary paths for all medium changers that are configured on the system, issue the cat /proc/scsi/IBMchanger command. The following example is taken from the IBMchanger file:

 Number     Model          SN              HBA            FO Path
  3         03584L22       IBM1002345      qla2xxx        Primary
  4         03584L22       IBM1002345      qla2xxx        Alternate

The name of the special file that is associated with the primary path for this medium changer is /dev/IBMchanger3. Specify /dev/IBMchanger3 as the value of the DEVICE parameter in the DEFINE PATH command for this device.

To display path-related details about a particular tape drive on the system, use the itdt -f /dev/IBMtapex path command, where x is the number of a configured tape device. To display path-related details about a particular medium changer on the system, use the itdt -f /dev/IBMchangerx path command, where x is the number of a configured medium changer.

On Windows, multipath I/O for medium changers and tape drives is not enabled automatically when the device driver is installed. For instructions about configuring multipath I/O, see the IBM Tape Device Drivers Installation and User's Guide. If multipath I/O is configured, a device has two matching device names with different locations. To obtain detailed information about the primary path and the alternate path, run the IBM Tape Diagnostic Tool with the qrypath function. The output is similar to the following example:

C:\Users\Administrator\Downloads\ITDT> .\itdt.exe qrypath -f \\.\Tape0
Querying SCSI paths...
Total paths configured........... 2

Alternate Path
Logical Device................. Tape0
Serial Number.................. 0000078F7612
SCSI Host ID.................... 8
SCSI Channel................... 0
Target ID.......................... 3
Logical Unit....................... 0
Path Enabled................... Yes

Primary Path
Logical Device................. Tape0
Serial Number.................. 0000078F7612
SCSI Host ID................... 8
SCSI Channel.................. 0
Target ID......................... 1
Logical Unit...................... 0
Path Enabled................... Yes


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