14 Implementing the IBM General Parallel File System (GPFS) in a Cross-Platform Environment
System p has two options for multipath configuration:
With MPIO, only one path per controller is activated.
With the IBM Subsystem Device Driver Path Control Module (SDDPCM), the full path is
activated but cannot be used over four-path.
Multipath driver RDAC and DM-Multipath driver on Linux
The multipath driver RDAC and the DM-multipath driver for Linux are available as follows:
RDAC is provided by the IBM DS3000, DS4000®, DS5000 series storage solutions.
The DM-multipath driver is provided by the open source community.
Multipath drivers provide solutions to the following functions:
– HBA failure
– FC cable failure
– SAN switch failure
– Array controller port failure
The Redundant Disk Array Controller (RDAC) component contains a multipath driver and
hot-add support. It must be installed on the host system, and it provides redundant paths to
the storage server when both RAID controllers are installed. If a RAID controller fails or
becomes inaccessible because of connectivity problems, RDAC reroutes the I/O requests
through another RAID controller. The hot-add part of RDAC enables you to dynamically
register new logical drives to the operating system.
To configure RDAC, you must disable the auto volume transfer (AVT) function and the
1. An important option on RDAC is to use the following parameter, which disables the
failback function on the storage side (the parameter is in the /etc/mpp.conf file):
2. Next, use the HBA
driver failover time minimize option. For example, if you have QLogic
HBA, use the following parameter to minimize failover time (the parameter is in the
Also consider enabling on the RDAC and the HBA driver modules the failover options.
DM-Multipath can provide failover in an active-passive configuration. In an active-passive
configuration, only half the paths are used at any time for I/O. If any element of an I/O path
(the cable, switch, or controller) fails, DM-Multipath switches to an alternate path. It can be
configured in active-active mode, where I/O is spread over the paths in a round-robin fashion.
In certain configurations, DM-Multipath can detect loading on the I/O paths and dynamically
rebalance the load. With DM-Multipath configured, a failure at any of these points causes
DM-Multipath to switch to the alternate I/O path.
Important: RDAC must be loaded, even if you have only one HBA on the host.