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

• Persistent device settings: All the device settings such as load balancing policies, path grouping
policies are persistent across reboots and SAN reconfigurations.
• Device exclusion: Provides device exclusion feature through blacklisting of devices.
• Path monitoring: Periodically monitors each path for status and enables faster failover and failback.
• Online device addition and deletion: Devices can be added to or deleted from Device Mapper
(DM) Multipath without rebooting the server or disrupting other devices or applications.
• Management Utility: Provides Command Line Interface (CLI) to manage Multipath devices.
• Boot from SAN: Provides multipathing for operating system installation partitions on SAN devices.
• Cluster support: Provides multipathing in HP Serviceguard and SteelEye LifeKeeper clustering en-
vironment.
• Volume Manager support: Provides support for multipathing devices to be configured under Logical
Volume Manager.
Software components
This section discusses the following software components of Device Mapper Multipath:
• dm-multipath kernel module -- Routes I/O and provides failover to paths and path groups.
• multipath configuration tool -- Provides commands to configure, list, and flush Multipath devices.
• multipathd daemon -- Monitors path status. When paths revert, multipathd daemon may also
initiate path group switches to ensure that the optimal path group is used.
• kpartx utility -- Reads partition tables on the specified device and creates device maps over the
detected partitions. The kpartx utility is called from hotplug whenever device maps are created
and deleted.
• devmap-name -- Provides a meaningful device name to udev for device maps (devmaps).
DM Multipath operation
DM Multipath enables hosts to route I/O over the multiple paths available to an end storage unit
(LUN). A path refers to the connection from an HBA port to a storage controller port. When an active
path through which I/O happens fails, DM Multipath reroutes the I/O over other available paths.
On a Linux host, when there are multiple paths to a storage controller, each path appears as a
separate block device and hence results in multiple block devices for single LUN. DM Multipath creates
a new Multipath block device for those devices that have the same LUN WWN.
For example, a host with two HBAs when attached to a storage controller with two ports through a
single FC switch provides four block devices: /dev/sda, /dev/sdb, /dev/sdc, and /dev/sdd.
DM Multipath creates a single block device, /dev/mapper/mpath1, that reroutes I/O through these
four underlying block devices.
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