Guidelines for OpenVMS Cluster Configurations


Previous Contents Index


Chapter 6
Configuring Multiple Paths to SCSI and Fibre Channel Storage

This chapter describes multipath SCSI support, and applies to disks and tapes except where noted. The SCSI protocol is used on both the parallel SCSI interconnect and the Fibre Channel interconnect. The term SCSI is used to refer to either parallel SCSI, or Fibre Channel (FC) devices throughout the chapter.

Note

OpenVMS Alpha Version 7.3-1 introduced support for failover between local and MSCP served paths to SCSI disk devices. This capability is enabled by the MPDEV_REMOTE system parameter setting of 1, which is the default setting. This type of failover does not apply to tape devices.

This SCSI multipath feature may be incompatible with some third-party disk caching, disk shadowing, or similar products. Specifically, third-party products that rely on altering the Driver Dispatch Table (DDT) of either the OpenVMS Alpha SCSI disk class driver (SYS$DKDRIVER.EXE), the OpenVMS Alpha SCSI tape class driver (SYS$MKDRIVER.EXE), or the SCSI generic class driver (SYS$GKDRIVER) may need to be modified in order to function correctly with the SCSI multipath feature. HP advises that you not use such software on SCSI devices that are configured for multipath failover (for example, SCSI devices that are connected to HSZ70 and HSZ80 controllers in multibus mode) until this feature is supported by the producer of the software. HP offers driver dispatch table (DDT) routines for modifying these third-party products to make them compatible with SCSI multipath failover.

See Section 6.2 for important requirements and restrictions for using the multipath SCSI function.

Note that the Fibre Channel and parallel SCSI interconnects are shown generically in this chapter. Each is represented as a horizontal line to which the node and storage subsystems are connected. Physically, the Fibre Channel interconnect is always radially wired from a switch, as shown in Figure 7-1. Parallel SCSI can be radially wired to a hub or can be a daisy-chained bus.

The representation of multiple SCSI disks and SCSI buses in a storage subsystem is also simplified. The multiple disks and SCSI buses, which one or more HSZx, HSGx, or HSVx controllers serve as a logical unit to a host, are shown in the figures as a single logical unit.

The following topics are presented in this chapter:

6.1 Overview of Multipath SCSI Support

A multipath SCSI configuration provides failover from one path to a device to another path to the same device. Multiple paths to the same device increase the availability of that device for I/O operations. Multiple paths also offer higher aggregate performance. Figure 6-1 shows a multipath SCSI configuration. Two paths are configured from a computer to the same virtual storage device.

Multipath SCSI configurations for disk devices can use either parallel SCSI or Fibre Channel as the storage interconnect, as illustrated by Figure 6-1. Multipath SCSI configurations for tape devices can use only Fibre Channel as the storage interconnect.

Two or more paths to a single device are called a multipath set. When the system configures a path to a device, it checks for an existing device with the same name but a different path. If such a device is found, and multipath support is enabled, the system either forms a multipath set or adds the new path to an existing set. If multipath support is not enabled, then no more than one path to a device is configured.

The system presents a multipath set as a single device. The system selects one path to the device as the "current" path, and performs all I/O over this path until there is a failure or the system manager requests that the system switch to another path.

Multipath SCSI support provides the following types of failover:

Direct SCSI to direct SCSI failover requires the use of multiported SCSI devices. Direct SCSI to MSCP served failover requires multiple hosts per SCSI bus, but does not require multiported SCSI devices. These two failover types can be combined. Each type and the combination of the two are described next.

6.1.1 Direct SCSI to Direct SCSI Failover

Direct SCSI to direct SCSI failover can be used on systems with multiported SCSI devices. The dual HSZ70, the HSZ80, the HSG80, the dual MDR, and the HSV110 are examples of multiported SCSI devices. A multiported SCSI device can be configured with multiple ports on the same physical interconnect so that if one of the ports fails, the host can continue to access the device through another port. This is known as transparent failover mode and has been supported by OpenVMS for disk devices since Version 6.2.

OpenVMS Version 7.2 introduced support for a new failover mode in which the multiported disk device can be configured with its ports on different physical interconnects. This is known as multibus failover mode.

The HSx failover modes are selected by HSx console commands. Transparent and multibus modes are described in more detail in Section 6.3.

Figure 6-1 is a generic illustration of a multibus failover configuration.

Note

Configure multiple direct SCSI paths to a disk device only when multipath support is enabled on all connected nodes, and the HSZ/G is in multibus failover mode.

The two logical disk devices shown in Figure 6-1 represent virtual storage units that are presented to the host by the HSx controller modules. Each logical storage unit is "on line" to one of the two HSx controller modules at a time. When there are multiple logical units, they can be on line to different HSx controllers so that both HSx controllers can be active at the same time.

In transparent mode, a logical unit switches from one controller to the other when an HSx controller detects that the other controller is no longer functioning.

In multibus mode, as shown in Figure 6-1, a logical unit switches from one controller to the other when one of the following events occurs:

Figure 6-1 Multibus Failover Configuration


Note the following about Figure 6-1:

The multibus configuration offers the following advantages over transparent failover:

In a multibus failover configuration, SAS and its controllers (LSI 1068 and LSI 1068e) can also be used.

6.1.2 Direct SCS to MSCP Served Failover (Disks Only)

OpenVMS provides support for multiple hosts that share a SCSI bus. This is known as a multihost SCSI OpenVMS Cluster system. In this configuration, the SCSI bus is a shared storage interconnect. Cluster communication occurs over a second interconnect (LAN).

Multipath support in a multihost SCSI OpenVMS Cluster system enables failover from directly attached SCSI storage to MSCP served SCSI storage, as shown in Figure 6-2.

Figure 6-2 Direct SCSI to MSCP Served Configuration With One Interconnect


Note the following about this configuration:

Multipath support in such a multihost SCSI OpenVMS Cluster system also enables failover from MSCP served SCSI storage to directly attached SCSI storage. For example, the following sequence of events can occur on the configuration shown in Figure 6-2:

Note

In this document, the capability to fail over from direct SCSI to MSCP served paths implies the ability to fail over in either direction between direct and served paths.

In a direct SCSI to MSCP served failover configuration, SAS and its controllers (LSI 1068 and LSI 1068e) can also be used.

6.1.3 Configurations Combining Both Types of Multipath Failover

In a multihost SCSI OpenVMS cluster system, you can increase disk storage availability by configuring the cluster for both types of multipath failover (direct SCSI to direct SCSI and direct SCSI to MSCP served SCSI), as shown in Figure 6-3.

Figure 6-3 Direct SCSI to MSCP Served Configuration With Two Interconnects


Note the following about this configuration:

This configuration provides the advantages of both direct SCSI failover and direct to MSCP served failover.

In this type of configuration, SAS and its controllers (LSI 1068 and LSI 1068e) can also be used.

6.2 Configuration Requirements and Restrictions

The requirements for multipath SCSI and FC configurations are presented in Table 6-1.

Table 6-1 Multipath SCSI and FC Configuration Requirements
Component Description
Host adapter For parallel SCSI, the KZPBA-CB must be used. It is the only SCSI host adapter that supports disk multipath failover on OpenVMS. For Fibre Channel, both the KGPSA-BC and the KGPSA-CA support multipath failover on OpenVMS.
Alpha console firmware For systems with HSZ70 and HSZ80, the minimum revision level is 5.3 or 5.4, depending on your AlphaServer. For systems with HSG80, the minimum revision level is 5.4
Controller firmware For HSZ70, the minimum revision level is 7.3; for HSZ80, it is 8.3; for HSG80, it is 8.4. For MDR, the minimum revision level is 1170.
Controller module mode Must be set to multibus mode (disks only). The selection is made at the HS x console.
Full connectivity All hosts that are connected to an HS x in multibus mode must have a path to both HS x controller modules. This is because hosts that are connected exclusively to different controllers will switch the logical unit back and forth between controllers, preventing any I/O from executing.

To prevent this from happening, always provide full connectivity from hosts to controller modules. If a host's connection to a controller fails, then take one of the following steps to avoid indefinite path switching:

  • Repair the connection promptly.
  • Prevent the other hosts from switching to the partially connected controller. This is done by either disabling switching to the paths that lead to the partially connected controller (see Section 6.7.11), or by shutting down the partially connected controller.
  • Disconnect the partially connected host from both controllers.
Allocation classes For parallel SCSI, a valid HSZ allocation class is required (see Section 6.5.3). If a SCSI bus is configured with HSZ controllers only, and all the controllers have a valid HSZ allocation class, then it is not necessary to adhere to the older SCSI device naming rules for that bus. That is, the adapters require neither a matching port allocation class nor a matching node allocation class and matching OpenVMS adapter device names.

However, if there are non-HSZ devices on the bus, or HSZ controllers without an HSZ allocation class, you must follow the standard rules for shared SCSI buses for node and port allocation class assignments and for controller device names.

Booting from devices with an HSZ allocation class is supported on all AlphaServers that support the KZPBA-CB except for the AlphaServer 2 x00(A).

The controller allocation class is not used for FC devices.

Note: If you are using Volume Shadowing for OpenVMS, every disk must have a nonzero allocation class. All FC disks attached to HSG and HSV controllers are automatically assigned the allocation class value of 1, which satisfies the volume shadowing requirement.

The restrictions for multipath FC and SCSI configurations are presented in Table 6-2.

Table 6-2 Multipath FC and SCSI Configuration Restrictions
Capability Description
Devices supported DKDRIVER disk devices attached to HSZ70, HSZ80, HSG60, HSG80, and HSV110 controller modules are supported. MKDRIVER tapes and GKDRIVER tape robots attached to an MDR are supported. Other device types, such as tapes, and generic class drivers, such as GKDRIVER, are not supported.

Note that under heavy load, a host-initiated manual or automatic switch from one controller to another may fail on an HSZ70 or HSZ80 controller. Testing has shown this to occur infrequently. This problem has been fixed for the HSZ70 with the firmware HSOF V7.7 (and later). The problem will be fixed for the HSZ80 in a future release. This problem does not occur on HSG x or HSV x controllers, nor on the MDR.

Mixed-version and mixed-architecture clusters All hosts that are connected to an HSZ, HSG, or HSV in multibus mode must be running OpenVMS Version 7.2 or higher.

To use multipath failover to a served path, MPDEV_REMOTE must be enabled on all systems that have direct access to shared SCSI or Fibre Channel devices.

SCSI to MSCP failover
MSCP to SCSI failover
Multiple hosts must be attached to SCSI disk devices via a shared SCSI bus (either parallel SCSI or Fibre Channel). The MPDEV_REMOTE system parameter must be set to 1 on these hosts.
Volume Shadowing for OpenVMS The use of default settings for certain system parameters may lead to the occasional removal of shadow set members that are configured for multipath support. The shadow set members where this has been observed are using Volume Shadowing for OpenVMS.

Therefore, when configuring multipath shadow sets using Volume Shadowing for OpenVMS, observe the following recommended setting for these system parameters:
System Parameter Recommended Setting
MSCP_CMD_TMO 60 as a minimum.
The value of 60 is appropriate for most configurations. Some configurations may require a higher setting.
SHADOW_MBR_TMO At least 3 x MSCP_CMD_TMO.
SHADOW_SYS_TMO At least 3 x MSCP_CMD_TMO.
MVTIMEOUT At least 4 x SHADOW_MBR_TMO.

6.3 HSx Failover Modes

The HSZ70, HSZ80, and HSGx implement two modes of failover operation when they are in a dual-redundant configuration, transparent failover mode, and multibus failover mode. HSVx supports multibus failover only.

Note

Starting with OpenVMS Alpha Version 7.3, transparent failover mode for the HSGx is supported.

For the system to operate correctly, the HSx failover mode must be compatible with the configuration of the interconnect hardware and the host operating system software, as described in the following sections.

6.3.1 Transparent Failover Mode

In transparent failover mode, the HSx presents each logical unit on one port of the dual controller pair. Different logical units may be assigned to different ports, but an individual logical unit is accessible through one port at a time. As shown in Figure 6-4, when the HSZ detects that a controller module has failed, it moves the logical unit to the corresponding port on the surviving controller.

Figure 6-4 Storage Subsystem in Transparent Mode


The assumption in transparent mode is that the two ports are on the same host bus, so the logical unit can move from one port to the other without requiring any changes to the host's view of the device. The system manager must ensure that the bus configuration is correct for this mode of failover. OpenVMS has supported transparent failover for HSZ controllers since Version 6.2.

To select transparent failover mode on the HSZ or HSG, enter one of the following commands at the console, depending on your configuration:


HSZ> SET FAILOVER COPY=THIS_CONTROLLER 

or


HSZ> SET FAILOVER COPY=OTHER_CONTROLLER 

An example of the output of a console SHOW command on an HSZ in transparent mode follows:


z70_A => SHOW THIS_CONTROLLER 
Controller: 
        HSZ70 ZG64100160 Firmware XB32-0, Hardware CX25 
        Configured for dual-redundancy with ZG64100136 
            In dual-redundant configuration 
        Device Port SCSI address 7 
        Time: 02-DEC-1998 09:22:09 
Host port: 
        SCSI target(s) (0, 2, 3, 4, 5, 6) 
        Preferred target(s) (3, 5) 
        TRANSFER_RATE_REQUESTED = 20MHZ 
        Host Functionality Mode = A 
        Allocation class           0 
        Command Console LUN is target 0, lun 1 
Cache: 
        32 megabyte write cache, version 4 
        Cache is GOOD 
        Battery is GOOD 
        No unflushed data in cache 
        CACHE_FLUSH_TIMER = DEFAULT (10 seconds) 
        NOCACHE_UPS 

6.3.2 Multibus Failover Mode (Disks Only)

In multibus failover mode, the HSx responds to SCSI Inquiry commands from the host on all ports of the dual controller pair. This allows the host to be aware of all the possible paths to the device. There are two advantages to having the host aware of all the paths to a device:

Note that, although the logical unit is visible on all ports, it is on line and thus capable of doing I/O on the ports of only one controller at a time. Different logical units may be on line to different controllers, but an individual logical unit is on line to only one controller at a time, as shown in Figure 6-5.

Figure 6-5 Storage Subsystem in Multibus Mode


You can determine which controller a logical unit is on line to by entering the HSx console command, as follows:


z70_A => SHOW UNIT FULL 
    LUN                                      Uses 
-------------------------------------------------------------- 
  D200                                       DISK20300 
        Switches: 
          RUN                    NOWRITE_PROTECT        READ_CACHE 
          MAXIMUM_CACHED_TRANSFER_SIZE = 32 
          ACCESS_ID = ALL 
        State: 
          ONLINE to the other controller 
          PREFERRED_PATH = OTHER_CONTROLLER 
        Size: 2050860 blocks 

The host executes I/O to a logical unit on one path at a time, until that path fails. If a controller has two ports, then different hosts can access the same logical unit over different ports of the controller to which the logical unit is on line.

An HSx in multibus failover mode can only be used with the multipath functionality introduced in OpenVMS Version 7.2.

To select multibus failover mode, enter one of the following commands at the HSx: console, whichever is appropriate to your configuration:


HSZ> SET MULTIBUS_FAILOVER COPY=THIS_CONTROLLER 

or


HSZ> SET MULTIBUS_FAILOVER COPY=OTHER_CONTROLLER 

An example of the output of a console SHOW command on an HSx controller in multibus mode follows:


z70_B => SHOW THIS_CONTROLLER 
Controller: 
        HSZ70 ZG64100136 Firmware XB32-0, Hardware CX25 
        Configured for MULTIBUS_FAILOVER with ZG64100160 
            In dual-redundant configuration 
        Device Port SCSI address 6 
        Time: NOT SET 
Host port: 
        SCSI target(s) (0, 2, 3, 4, 5, 6) 
 
        TRANSFER_RATE_REQUESTED = 20MHZ 
        Host Functionality Mode = A 
        Allocation class           0 
        Command Console LUN is target 0, lun 1 
Cache: 
        32 megabyte write cache, version 4 
        Cache is GOOD 
        Battery is GOOD 
        No unflushed data in cache 
        CACHE_FLUSH_TIMER = DEFAULT (10 seconds) 
        NOCACHE_UPS 


Previous Next Contents Index