HP StorageWorks Device Mapper Multipath Enablement Kit for HP StorageWorks Disk Arrays installation and reference guide (AA-RW8RE-TE, June 2009)

1 Introduction
This chapter addresses the following topics:
•Overview
• Features
• Software components
• DM Multipath operation
Overview
HPDM Multipath is an enablement kit for HP StorageWorks Disk Arrays based on Device Mapper
Multipath, included in the Linux operating system distributions. Device Mapper is an infrastructure in
the Linux kernel. It provides a generic way to create virtual layers of block devices. It supports striping,
mirroring, snapshots, concatenation, and multipathing. The multipath feature is provided with
combination of DM Multipath kernel modules and multipath-tools user-space package.
Features
Device Mapper Multipath offers the following features:
• I/O failover and failback: Provides transparent failover and failback of I/Os by rerouting I/Os
automatically to an alternative path when a path failure is sensed, and routing them back when
the path is restored.
• Path grouping policies: Paths are coalesced based on the following path-grouping policies:
• Priority based path-grouping
— Provides priority to group paths based on Asymmetric Logical Unit Access (ALUA) state
— Provides static load balancing policy by assigning higher priority to the preferred path
• Multibus — All paths are grouped under a single path group
• Group by serial — Paths are grouped together based on controller serial number
• Failover only — Provides failover without load balancing by grouping the paths into individual
path groups
• I/O load balancing policies: Provides the following load balancing policies within a path group:
• Weighted round robin — This round-robin algorithm routes rr_min_io number of I/Os on
a selected path before switching to the next path.
• Least pending I/O path — This determines the number of non-serviced requests pending on a
path and selects the path which has the least number of pending requests for service.
• DM service time — This is a service time oriented dynamic load balancer, which selects a path
to complete the incoming I/O with the shortest time.
• Device name persistence: Device names are persistent across reboots and Storage Area Network
(SAN) reconfigurations. Device Mapper also provides configurable device name aliasing feature
for easier management.
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