IBM N Series Hardware Manual page 279

With the implicit ALUA style, the host multipathing software can monitor the path states but
cannot change them, either automatically or manually. Of the active paths, a path can be
specified as preferred (optimized in T10), and as non-preferred (non-optimized). If there are
active preferred paths, only those paths receive commands and are load balanced to evenly
distribute the commands. If there are no active preferred paths, the active non-preferred paths
are used in a round-robin fashion. If there are no active non-preferred paths, the LUN cannot
be accessed until the controller activates its standby paths.
Tip: Generally, use ALUA on hosts that support ALUA.
Verify that a host supports ALUA before implementing, because otherwise a cluster failover
might result in system interruption or data loss. All N series LUNs presented to an individual
host must have ALUA enabled. The host's MPIO software expects ALUA to be consistent for
all LUNs with the same vendor.
Traditionally, you had to manually identify and select the optimal paths for I/O. Utilities such as
dotpaths for AIX are used to set path priorities in environments where ALUA is not supported.
Using ALUA, the administrator of the host computer does not need to manually intervene in
path management. It is handled automatically. Running MPIO on the host is still required, but
no additional host-specific plug-ins are required.
This process allows the host to maximize I/O by using the optimal path consistently and
automatically.
ALUA has the following limitations:
ALUA can only be enabled on FCP initiator groups.
ALUA is not available on non-clustered storage systems for FCP initiator groups.
ALUA is not supported for iSCSI initiator groups.
To enable ALUA on existing non-ALUA LUNs, perform these steps:
1. Validate the host OS and the multipathing software as well as the storage controller
software support ALUA. For example, ALUA is not supported for VMware ESX until
vSphere 4.0. Check with the host OS vendor for supportability.
2. Check the host system for any script that might be managing the paths automatically and
disable it.
3. If using SnapDrive, verify that there are no settings that disable the ALUA set in the
configuration file.
ALUA is enabled or disabled on the igroup mapped to a LUN on the N series controller. The
default ALUA setting in Data ONTAP varies by version and by igroup type. Check the output of
the igroup show -v <igroup name> command to confirm the setting.
Enabling ALUA on the igroup activates ALUA.
Chapter 19. Host multipathing
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