8x00 family of Disk Arrays 3.02.00 (AA-RW7SL-TE, March 2009)

4 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 3 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 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.

• 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 5 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.

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