IBM N Series, Hardware Manual

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Chapter 19. Host multipathing
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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.
Verify that a host supports ALUA before it is implemented because a cluster failover might
result in system interruption or data loss. All N series LUNs that are 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 manually identified and selected 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.
By using ALUA, the administrator of the host computer does not need to manually intervene
in path management. Instead, it is handled automatically. Running MPIO on the host is still
required, but no other 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:
򐂰 Can be enabled only on FCP initiator groups
򐂰 Is unavailable on non-clustered storage systems for FCP initiator groups
򐂰 Is not supported for iSCSI initiator groups
To enable ALUA on existing non-ALUA LUNs, complete the following steps:
1. Validate the host OS and the multipathing software and 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 SnapDrive is used, verify that there are no settings that disable the ALUA set in the
configuration file.
ALUA is enabled or disabled on the igroup that is 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.
Tip: Generally, use ALUA on hosts that support ALUA.