HP MPIO Full Featured DSM for P6x00 family of Disk Arrays 4.04.00 Installation and Reference Guide (757324-001, May 2014)

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
Shortest Queue Service Time (SQST)
the least Shortest Queue Service Time (SQST) outstanding
I/O service time for the incoming I/O request. This is the
default policy set by the DSM.
This policy selects Shortest Queue Requests (SQR) the path
with least outstanding I/O requests.
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.
Subset based load balancing 13