HiCommand Dynamic Link Manager (HDLM) for Windows Systems User's Guide (HIT5201-96001, January 2008)

24 Chapter 2 HDLM Functions
2.6.1.1 Using TagmaStore AMS/WMS Series, Thunder 9500V Series, or Thunder 9200 Series
HDLM performs load balancing between owner paths or between non-owner paths. An owner
path is a path that passes through the channel adapter. This path is set on the owner
controller of the storage system LU. When you set an LU, you determine the owner
controller and non-owner controller for the LU. Since the owner controller varies depending
on the LU, the owner path also varies depending on the LU. A non-owner path is a path that
uses a channel adapter other than the owner controller (a non-owner controller). To prevent
performance in the entire system from deteriorating, HDLM does not perform load balancing
between owner paths and non-owner paths. When some owner paths cannot be used due to
a problem such as a failure, load balancing is performed among the remaining usable owner
paths. When no owner paths can be used, load balancing is performed among the non-owner
paths.
For the example in
Figure 2.9, suppose that the owner controller of LU0 is CHA0. When the
LU is accessed, the load is balanced between the paths (A) and (B) (that is, between owner
paths). When the path (A) cannot be used due to a problem such as a failure, the LU can
only be accessed via the path (B). When the paths (A) and (B) cannot be used, the load is
balanced between the paths (C) and (D) (that is, between non-owner paths).
2.6.1.2 When using the Lightning 9900 Series, Lightning 9900V Series, TagmaStore USP or
Universal Storage Platform V
All the online paths are owner paths. Therefore, for the example in Figure 2.8, when the LU
is accessed, the load is balanced among the paths (A), (B), (C), and (D). When one of the
paths cannot be used due to a problem such as a failure, the load is balanced among the
remaining paths.
2.6.2 Algorithms for Load Balancing
HDLM features the following two algorithms for load balancing:
 Round robin
Round robin distributes all I/Os among multiple paths.
 Extended round robin
Extended round robin distributes I/Os to paths depending on the type of the I/O, which
can be either sequential access or random access. For sequential access, I/Os are issued
to a single path. For random access, I/Os will be distributed to multiple paths.
Table 2.5 describes the types of load balancing (round robin and extended round robin) for
each I/O operation type.