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

Symantec DMP yes yes no yes

yes

HP Storage Works

SecurePath®

yes yes yes no no

EMC PowerPath® yes yes yes no no

Volume Manager based

Serviceguard supports two different volume managers which also provide a multi-pathing solution. If

customers choose LVM or SLVM as their volume manager, protection against path failures is provided

by failover to alternate links using LVM PVlinks (a.k.a. Alternate Links). If they choose VxVM or CVM,

Symantec Veritas Dynamic Multipathing (DMP) protects against path failures and provides additional

I/O load balancing options.

LVM PVlinks

LVM supports PVlinks as path management to a device to allow continued access to the device if the

primary I/O link fails. Such a device must have multiple active paths from the host. In HP-UX 11i v2,

LVM PVlink’s multi-pathing solution increases the data availability by providing automatic path

failover but does not provide I/O load balancing across multiple paths simultaneously.

LVM uses the device files created during HP-UX I/O boot phase in /dev/rdsk and /dev/dsk

directories (e. g.: /dev/dsk/c6t0d2 and /dev/dsk/c8t0d2). The path named first at volume

group creation time becomes the primary path. All paths added later become alternate paths. If the

primary path fails, LVM automatically re-routes I/Os to an alternate path. After a failed primary path

becomes available again, LVM automatically switches back to it.

A good practice is to manually distribute the load statically between multiple links at volume group

creation time. This is achieved by alternating the paths when naming the physical volumes. Figure 2

shows an example in which 4 devices are accessible through controller c6 and c8.

Figure 2: Static load balancing with PVlinks

vgcreate vgdb /dev/dsk/c6t0d0 /dev/dsk/c8t0d1 /dev/dsk/c6t0d2 /dev/dsk/c8t0d3

vgextend vgdb /dev/dsk/c8t0d0 /dev/dsk/c6t0d1 /dev/dsk/c8t0d2 /dev/dsk/c6t0d3

vgdisplay -v vgdb | grep "PV Name"

PV Name /dev/dsk/c6t0d0

PV Name /dev/dsk/c8t0d0 Alternate Link

PV Name /dev/dsk/c8t0d1

PV Name /dev/dsk/c6t0d1 Alternate Link

PV Name /dev/dsk/c6t0d2

PV Name /dev/dsk/c8t0d2 Alternate Link

PV Name /dev/dsk/c8t0d3

PV Name /dev/dsk/c6t0d3 Alternate Link

The PVlinks feature comes with LVM at no additional charge and works with all devices supported by

Serviceguard, but only for those that are configured to be under the control of LVM.

VxVM/CVM Dynamic Multi-pathing (DMP)

Symantec Veritas DMP increases the data availability and I/O performance by dynamically load

balancing I/O requests across multiple storage paths for faster throughput. There are several different

load balancing policies available. Chapter 2 and 3 of VERITAS Volume Manager 4.1 Administrator’s

Guide (HP-UX 11i v2) give an overview of DMP and describe the administration of it. The Dynamic

DMP is only available with arrays supported by VxVM/CVM; some arrays might require an ASL for support