Storage Multi-Pathing choices in HP-UX Serviceguard environments, August 2009

Multipathing white paper goes into further details about the different load balancing policies and

interoperability with other multi-pathing solutions.

VxVM’s configuration daemon creates its own path-independent device files in /dev/vx/rdmp and

/dev/vx/dmp at boot up time. It uses one device file per LUN or disk – not per path. Multiple paths

(SUBPATH) are associated with VxVM/DMP devices files. Figure 3 shows how to identify which paths

are associated with a specific device. In the example below, the disk group “dbDG” consists of two

subdisks “dbDG01” and “dbDG02” which reside on disk “c6t0d0” and “c6t0d1” (DANAME column).

Each of these disks has two active paths – one through c6 and the other through c8 (SUBPATH

column).

Figure 3: VxVM device file to subpath mapping

# vxdisk -g dbDG path

SUBPATH DANAME DMNAME GROUP STATE

c6t0d0 c6t0d0 dbDG01 dbDG ENABLED

c8t0d0 c6t0d0 dbDG01 dbDG ENABLED

c6t0d1 c6t0d1 dbDG02 dbDG ENABLED

c8t0d1 c6t0d1 dbDG02 dbDG ENABLED

The column “SUBPATH” lists HP-UX device files (paths) in directory “/dev/(r)dsk” and the column

“DANAME” lists VxVM/DMP device files from “/dev/vx/(r)dmp” directory. VxVM 4.1 also offers

the option to use “enclosure-based” naming schema for VxVM device files. With enclosure-based

naming the “DANAME” column could have entries like XP12K0_1 and XP12K0_2, or EVA80000_1

and EVA80000_2 instead of c6t0d0 and c6t0d1.

DMP is tightly integrated with VxVM and CVM. As part of HP’s Serviceguard Storage Management

Suite, DMP comes at no additional charge and works with most storage devices supported with

Serviceguard and VxVM/CVM. The VERITAS Volume Manager 4.1 Hardware Notes (HP-UX 11i v2) and

Device Support for Veritas Products on HP-UX provide details about the disk array configurations

supported with DMP.

Active/active arrays that fulfill certain requirements are supported without any additional software.

However, for those arrays that require more specialized handling, DMP’s architecture provides for

array-specific array support libraries (ASLs) for discovery and configuration and kernel mode array

policy modules (APMs) to perform array-specific functions in the I/O path. These libraries/modules

allow DMP to also support some active/passive storage systems in a cluster. ASLs and APMs for

arrays that are already on the market are usually included in and ship with DMP. To provide the

ability to support new arrays as they become available, ASLs and APMs can be added later. New

ASLs can be downloaded from Symantec’s support website.

The vxddladm listsupport command lists all active ASLs installed on a system. It is a best

practice for DMP 4.1 and earlier versions to deactivate unused ASLs to improve the speed of the

device discovery process. A faster device discovery process also leads to a faster system boot and

cluster join time. An unused ASL is an ASL for an array which is not connected to the system.

Symantec refers to this process as ASL Tuning and strongly recommends it on all pre 5.0 releases of

DMP. The DMP 5.0 device discovery algorithms have been enhanced so that optimum boot time

performance is achieved out of the box, without any ASL Tuning.

Storage platform specific multi-pathing solutions

Multi-pathing software from storage array vendors differ from volume manager based solutions mainly

in two areas. First, they are independent from the volume managers which are providing a logical

view of the storage devices to users and applications. This can be seen as an advantage. Second,