OpenVMS Cluster Systems - OpenVMS Systems - HP

Cluster Storage Devices
6.4 MSCP I/O Load Balancing
6.4.1 Load Capacity
The load capacity ratings for the VAX and Alpha systems are predetermined
by Compaq. These ratings are used in the calculation of the available serving
capacity for MSCP static and dynamic load balancing. You can override these
default settings by specifying a different load capacity with the MSCP_LOAD
parameter.
Note that the MSCP server load-capacity values (either the default value or the
value you specify with MSCP_LOAD) are estimates used by the load-balancing
feature. They cannot change the actual MSCP serving capacity of a system.
A system’s MSCP serving capacity depends on many factors including its power,
the performance of its LAN adapter, and the impact of other processing loads.
The available serving capacity, which is calculated by each MSCP server as
described in Section 6.4.3, is used solely to bias the selection process when a
client system (for example, a satellite) chooses which server system to use when
accessing a served disk.
6.4.2 Increasing the Load Capacity When FDDI is Used
When FDDI is used instead of Ethernet, the throughput is far greater. To take
advantage of this greater throughput, Compaq recommends that you change the
server’s load-capacity default setting with the MSCP_LOAD parameter. Start
with a multiplier of four. For example, the load-capacity rating of any Alpha
system connected by FDDI to a disk can be set to 1360 I/O per second (4x340).
Depending on your configuration and the software you are running, you may
want to increase or decrease this value.
6.4.3 Available Serving Capacity
The load-capacity ratings are used by each MSCP server to calculate its available
serving capacity.
The available serving capacity is calculated in the following way:
Step Calculation
1 Each MSCP server counts the read and write requests sent to it and periodically converts
this value to requests per second.
2 Each MSCP server subtracts its requests per second from its load capacity to compute its
available serving capacity.
6.4.4 Static Load Balancing
MSCP servers periodically send their available serving capacities to the MSCP
class driver (DUDRIVER). When a disk is mounted or one fails over, DUDRIVER
assigns the server with the highest available serving capacity to it. (TMSCP
servers do not perform this monitoring function.) This initial assignment is called
static load balancing.
6.4.5 Dynamic Load Balancing (VAX Only)
Dynamic load balancing occurs only on VAX systems. MSCP server activity is
checked every 5 seconds. If activity to any server is excessive, the serving load
automatically shifts to other servers in the cluster.
Cluster Storage Devices 6–23