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Figure 13. Optimized ESX multipathing configuration with HP StorageWorks EVA
Implicit LUN transition
To optimize the host data paths and reduce I/O on the mirror ports, the EVA can automatically transfer the
management of a virtual disk to the array controller that receives the most read requests for that virtual disk.
This process is known as the implicit LUN transition. There are several Perl and PowerShell scripts available
on the Web to automate ESX path assignments to leverage this behavior. For a PowerShell script example,
see the VM junkie Blog. Once set on the ESX host no further configuration is required. (For example, setting
the virtual disk preferred controller with Command View EVA is not required for non-replicated LUNs). The
ESX multipath setting takes precedence. The Vdisks automatically switch over their managing controller on
the EVA to use the most optimized path.
Implicit LUN transition and Continuous Access
When using Continuous Access EVA to replicate EVA virtual disks, the implicit LUN transition is
automatically disabled for all members of an HP Continuous Access EVA DR group. Because Continuous
Access requires that all members of a DR group be managed by the same controller, it would be necessary
to move all members of the DR group if excessive proxy reads were detected on any member. This would
impact performance and create a proxy read situation for the other virtual disks in the DR group which in
turn may trigger another transition eventually leading to a path trashing scenario.
This is an important consideration when setting your ESX server multipath configuration. Without the support
of implicit LUN transition for the replicated Vdisks, incorrect ESX host settings may cause one or more virtual
disks in a DR group to experience excessive proxy reads. With this in mind, load balancing scripts should
be used carefully as they may establish a less than optimal configuration, causing unnecessary proxy reads.
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