Control Path, Data Path, and Path Failover

Control Path, Data Path, and Path Failover

Control Path (CP)

The Quantum Tape libraries i6000, i500, i80 & i40 all have the capability to present the Partition (medium changer) through an existing drive within the partition. This type of partition presentation to the SAN/Host is called Control Path (CP). When the partition is presented through a specific drive, that drive must also be zoned or connected to the host that needs to command the medium changer. The drive and medium changer share the same World Wide Port Name (WWPN) but their LUN ID’s are different. The Drive is assigned LUN 0 and the Medium Changer is assigned LUN 1. You can pick which drive will serve as the “Control Path” medium changer presenter but the drive must be part of the partition and only one drive can present the partition when the Control Path feature is used. Support of the Control Path feature is restricted to specific LTO drive vintages and brands based on the library type. Please reference the specific tape library User’s Guide for a listing of restrictions.

Control Path is a good feature to leverage for medium changer device presentation in simplified SAN or direct attach environments. The primary limitation for Control Path is that if the drive presenting the medium changer fails or is removed then the medium changer will also disappear at the host level. You would have to manually move the Control Path presentation to a different drive or repair the existing Control Path drive to get the medium changer device to be seen again at the OS level of the host.

Basic Control Path Failover (BCPF)

The Basic Control Path Failover (BCPF) feature is specific to HP branded LTO-5 and above drives. The BCPF feature allows the user to select a Primary and Secondary drive to leverage for presenting the Control Path (medium changer) to the SAN. This is facilitated using a Virtual WWPN for the Medium Changer and it is presented as a LUN 0 device to the host. The Primary BCPF drive will present two WWPN’s to the SAN. The first is the drives WWPN and associated LUN 0 drive device. The second is the virtual WWPN assigned to the Partition and its associated LUN 0 medium changer device. This virtual WWPN function requires that the SAN switch supports the N_Port ID Virtualization (NPIV) feature and it is enabled. In this scenario the Virtual WWPN of the medium changer will be presented through the primary drive until there is an FC link failure on that drive. When the FC link fails on the Primary drive the Virtual WWPN presentation is switch over to the Secondary drive. The WWPN and LUN of the medium changer are not changed but the drive it’s being presented through is changed. The BCPF feature is commonly used by many customers as it provides a failover robustness that can handle the primary control path drive failing or being removed and still have valid presentation of the medium changer to the SAN through the secondary drive. A key limitation to the BCPF feature is that the control path drives must be connected to the same SAN fabric. This is a limitation to the NPIV feature on the switch its self. If a customer wants to deploy a multi-fabric SAN which has totally segregated fabric’s the BCPF feature will not function properly when a failover event occurs.

Advanced Control Path Failover (ACPF)

The Advanced Control Path Failover (ACPF) feature is specific to HP branded LTO-6 and above drives. The ACPF feature will present the Partition (Medium changer) through all of the drives at the same time within the partition as a LUN 1 device. This type of LUN 1 device presentation is much the same as the standard Control Path feature but in the ACPF scenario the medium changer is presented through all of the drives within the partition. The actual failover functionality from one drive to the next is not performed by the library or drives but instead a dedicated device driver that is installed on the Host(s). When the Advanced Path Failover (APF) device driver is installed on the host, the driver software determines which medium changer “Path” is to be used at any given moment. If one “Path” fails it will automatically transition to the next available “Path”. The APF device driver software hides the redundant medium changer devices from the Operating System and only presents one instance of the medium changer to the OS. The primary advantage to using ACPF over BCPF is that the ACPF feature does support multi-fabric SAN attachment of the drives. If customers have completely segregated SAN fabrics for drive attachment they must use ACPF to allow the control path to fail over between fabrics.

Advanced Control Path (ACP)

The Advanced Control Path (ACP) feature is specific to IBM branded LTO-5 and above drives. The ACP feature will present the Partition (Medium changer) through all of the drives at the same time within the partition as a LUN 1 device. This type of LUN 1 device presentation is much the same as the standard Control Path feature but in the ACP scenario the medium changer is presented through all of the drives within the partition. The actual failover functionality from one drive to the next is not performed by the library or drives but instead a dedicated device driver that is installed on the Host(s). When the Advanced Path Failover (APF) device driver is installed on the host, the driver software determines which medium changer “Path” is to be used at any given moment. If one “Path” fails it will automatically transition to the next available “Path”. The APF device driver software hides the redundant medium changer devices from the Operating System and only presents one instance of the medium changer to the OS. The primary advantage to using ACP over BCPF is that the ACP feature does support multi-fabric SAN attachment of the drives. If customers have completely segregated SAN fabrics for drive attachment they must use ACP to allow the control path to fail over between fabrics. An added bonus to using the ACP feature is that it enables the Data Path failover functionality by default. All it requires is that both FC ports on the drive be attached to the SAN and their respective WWPN’s are zoned to the Host(s).

Multi Control Path (MCP)

The Multi Control Path (MCP) feature will function with both IBM and HP LTO-5 and above drives. The MCB feature will present the Partition (Medium changer) through all of the drives at the same time within the partition as a LUN 1 device. This type of LUN 1 device presentation is much the same as the standard Control Path feature but in the ACP scenario the medium changer is presented through all of the drives within the partition. The actual failover functionality from one drive to the next is not performed by the library or drives but instead by the application software on the Host(s). The Application Software will determine which medium changer “Path” is to be used at any given moment. The primary advantage to using MCP over BCPF or ACPF is that the MCP feature does support multi-fabric SAN attachment of the drives and you don’t need to use special device drivers on the host(s). The challenge with using this feature is that the application software now has to manage the complexity of seeing multiple instances of the same medium changer and choose which “Path” is best to use. Please reference your application software technical manuals to see if it supports the MCP feature.

Basic Data Path Failover (BDPF)

The Basic Data Path Failover (BDPF) feature will function with HP branded LTO-5 and above drives. The supported drives must also have dual FC ports with both of them connected to the SAN/Host. When the BDPF feature is enabled the FC 1 port is switched to the Primary/Active state and all communication to the drive and or medium changer is facilitated through only that port. The FC 2 port is switched to the Secondary/Passive state and no data is passed through the port. When the FC 1 port link fails it is transitioned to the Secondary/Passive state and the FC 1 WWPN and Device LUN(s) are passed over to the FC 2 port. Since the FC 2 port takes on the WWPN and Device LUN(s) there are no changes seen or needed on the SAN/Host to continue to use the drive/medium changer. The primary limitation to the BDPF feature is that you need to manually fail it over to properly Zone it on a multi-fabric SAN because the secondary port is in a Secondary/Passive mode until the Primary port fails

Advanced Data Path Failover (ADPF)

The Advanced Data Path Failover (ADPF) feature will function with HP branded LTO-6 and above drives. The supported drives must also have dual FC ports with both of them connected to the SAN/Host. The actual failover functionality from one FC port to the next is not performed by the library or drives but instead a dedicated device driver that is installed on the Host(s). When the Advanced Path Failover (APF) device driver is installed on the host, the driver software determines which FC Port “Path” is to be used at any given moment. If one “Path” fails it will automatically transition to the next available “Path”. The APF device driver software hides the redundant drive and medium changer devices from the Operating System and only presents one instance of the medium changer and drive to the OS. The primary advantage to using ADPF over BDPF is that the ACPF feature does support multi-fabric SAN attachment of the drives. If customers have completely segregated SAN fabrics for drive attachment they must use ADPF to allow the FC Port path to fail over between fabrics.

Summary

With many different Control or Data Path features to pick from it is up to the system administrator to determine which feature to leverage for their given deployment. It is easy to make the Control or Data path deployment far more complex than it really needs to be. The Standard or Basic features will work for the vast majority of customers. The primary reason to leverage the advanced features is if your SAN infrastructure is leveraging segregated fabrics with the same devices occupying both fabrics.

Some libraries require additional licensing to leverage specific Control/Data Path features. Please reference the Quantum Tape Library User’s Guide for more detailed information about the specific features and how to configure them.

Audience: 
Public Unrestricted
Review/Evaluate: 
2017-06-22
Document Type: