Datasheet

MPIO solution that provides redundant paths to a single set of LUNs. There are two MPIO

scenarios possible with VIOS that remove the requirement for two sets of LUNs:

 A single VIOS partition using two FC adapters to connect to the same set of LUNs

 Two VIOS partitions providing redundant paths to the same set of LUNs on a single open

storage subsystem

6.3.2.1 Subsystem Device Driver – Path Control Module (SDDPCM)

VIOS also supports the Subsystem Device Driver – Path Control Module (SDDPCM) for certain

storage subsystems. Examples of supported subsystems include the SAN Volume Controller

(SVC), Storwize V7000 and IBM DS8000 system. To find out whether a particular storage system

supports SDD-PCM for VIOS, refer to its interoperability matrix on the SDDPCM website:

https://www-304.ibm.com/support/docview.wss?uid=ssg1S4000201. Note that there are separate

support statements for AIX and VIOS. Always check if the VIOS version is supported by a

specific SDDPCM driver version, and that you download the correct SDDPCM driver for the

correct storage system and VIOS. If SDDPCM is supported on your storage subsystem for VIOS,

download and install the driver following the instructions in the Multi-path Subsystem Device

Driver User's Guide at the same location.

6.3.2.2 Path redundancy with a single VIOS

If a VIOS LPAR has two or more FC adapters assigned and the correct host configuration is

created in the open storage subsystem, VIOS will have redundant paths to the LUNs connected

to the FC adapters. VIOS includes a basic MPIO driver, which has been the default instead of the

RDAC (Redundant Disk Array Controller) driver since November 2008. The MPIO driver can be

used with any storage subsystem which VIOS supports and is included in a default install. In this

case, configuration is required only on the storage subsystem in order to connect a single set of

LUNs to both ports on a FC adapter owned by VIOS.

This multi-path method can be configured in two ways: round-robin, or failover. For storage

systems such as XIV, V7000 and DS8000 that support active-active multi-path connections,

either method is allowed. Path failover will use one path and leave the other idle until the first

path fails, while round-robin will always attempt to use both of the paths, if available, to maximize

throughput. For systems that only support active-passive connections, such as IBM System

Storage DS5000, DS4000, and DS3000 subsystems, failover is the only method allowed.

The access method can be configured on a per-LUN basis from the VIOS command line through

a Telnet or SSH session. To show the current multi-path algorithm, run the following command:

 lsdev –dev hdiskX –attr

 Look for the algorithm attribute value. Also look at the reserve_policy and

queue_depth attributes.

 When you have multiple paths to the same storage, the reserve_policy should be set to

no_reserve to allow either path to access the storage.

 The queue_depth tells VIOS how many concurrent I/O requests to send to the storage

controllers. Check the documentation for the storage subsystem to determine the correct

value to use. A large value can flood some of the entry level subsystems and cause

problems.

 For SVC or Storwize V7000: The attribute algorithm should be set as follows:

o If the driver SDDPCM is used in VIOS, the attribute should be set to

load_balance,

o If the MPIO (built in) driver is used the attribute should be set to round_robin.

To change the multi-path algorithm, run the following command: