HP MPIO Full Featured DSM for EVA4x00/6x00/8x00 family of Disk Arrays 4.01.00 installation and reference guide (AA-RW7SP-TE, September 2010)

3 Load balancing
Load Balancing is used to distribute I/O request across multiple paths to a logic unit for optimal usage
of paths and improve I/O performance. Depending on the load balancing policy set for a logic unit,
an appropriate path is selected based on the statistics over that path. The DSM currently supports the
following five load balance policies.
Table 2 Load balancing policies
This policy assigns equal priority to all the paths available
for a device and routes I/O request to each path in order.
Round Robin (RR)
This policy selects the path based on the service time for
the outstanding I/O requests and chooses the path with
the least outstanding I/O service time for the incoming I/O
request. This is the default policy set by the DSM.
Shortest Queue Service Time (SQST)
This policy selects the path with least outstanding I/O re-
quests.
Shortest Queue Requests (SQR)
This policy selects the path with least outstanding I/O bytes.Shortest Queue Bytes (SQB)
This policy does not implement any I/O load balancing.
However, the DSM selects a path to service all the I/O
requests if preferred path is not set.
No Load Balance (NLB)
NOTE:
• All devices that are part of the logical unit group have the same load balancing policy.
• Unique preferred path can be set for the devices that are part of the Logical Unit Group; that is,
different preferred paths can be set for members of a logical unit group as long as they are set
on the same controller.
• The load balancing policy for a given LUN is persistent across host system reboots.
You can modify the load balancing policy for a given LUN either through the HP MPIO DSM Manager
or HP DSM CLI. See Chapter 4 for more information.
Subset based load balancing
The HP MPIO DSM implements subset based load balancing algorithms that allows only a selected
set of paths to be used for I/O requests. The subset setting is supported on a per logical unit basis
and works in conjunction with the load balancing policies supported by the DSM.
By default, subset setting is disabled for the newly discovered LUN. If the newly discovered LUN is
part of any logical unit group, the subset setting of the existing group prevails.
HP MPIO Full Featured DSM for EVA4x00/6x00/8x00 family of Disk Arrays 4.01.00 installation
and reference guide 19