Round-Robin Load Balancing - VMware ESX Server 3.5, VMware ESX Server 3i version 3.5, VMware VirtualCenter 2.5

http://www.vmware.com/pdf/vi3_35_25_roundrobin.pdf

The following options are supported and discussed in more detail in Table 1.
esxcfg-mpath --lun [--policy [mru|rr|fixed|custom]]
[--custom-hba-policy|-H [mru|preferred|any|minq]] |
[--custom-target-policy|-T [mru|preferred|any]] |
[--custom-max-commands|-C ] |
[--custom-max-blocks|-B ]
Notes
If you set the custom-max-blocks and custom-max-commands, options, the system attempts to switch paths
as soon as one of the limits is reached.
If you set the target or the HBA policy to preferred, the system chooses the target or the HBA of the preferred
path when possible. If a preferred policy is set on an active/passive SAN array, and the preferred target is not
on the active SP (Storage Processor), the system does not select the preferred target but a target on the active SP.
Path switching is not performed if an outstanding SCSI reservation is on the target, or if a path failover is
underway. Path switching is delayed until an I/O request is performed when no reservations or path failovers
are pending.
Table 1. Policy Options for esxcfg-mpath
Option Description
--policy
-p
[mru|rr|fixed|custom]
Set the policy for a given LUN to mru, rr, fixed, or custom. This option requires that
the --lun option is also passed to indicate the LUN to set the policy for.
􀂄 Most recently used (mru) selects the path most recently used to send I/O to a device.
􀂄 Round robin (rr) uses the mru target selection policy and the any HBA selection
policy to select paths.
􀂄 Fixed (fixed) uses only the active path.
􀂄 Custom (custom) sets the LUN to expect a custom policy.
NOTE After you’ve set the policy to custom for a LUN, you can specify one or more of
the other options. For any LUN, you set the policy to custom only once.
--custom-hba-policy
-H
Selection policy for the HBA. Use one of the following:
􀂄 preferred — Use the HBA that’s part of the preferred path.
􀂄 mru – Use the most recently used HBA.
􀂄 minq – HBA that has the smallest number of outstanding I/O commands when the
ESX Server system does the path selection.
􀂄 any – Use any available HBA.
--custom-target-policy
-T
Selection policy for the target. Use one of the following:
􀂄 preferred – Use the target that is part of the preferred path.
􀂄 mru – Use the most recently used target.
􀂄 any – Use any available target.
--custom-max-commands
-C
Maximum number of I/O requests to be issued on a given path before the system tries
to select a different path. A setting of zero (0) disables switching based on commands.
Specify the number of commands as an integer. Default is 50.
--custom-max-blocks
-B
Maximum number of I/O blocks to be issued on a given path before the system tries to
select a different path. A setting of zero (0) disables switching based on blocks.
Specify the number of blocks as an integer. Default is 2048.