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14 Dell Storage PS Series Architecture: Load Balancers | TR1070
which is a composite score of factors such as RAID type, disk speed, number of disks, as well as PS Series
controller type and the current I/O load. The member with the lower relative busy-ness will be chosen for the
data exchange.
The APLB works well in a variety of environments. For example, in PS Series pools with members displaying
similar performance characteristics, the net effect is to eliminate hot spots in the pool. This is shown in Figure
1. In pools with members displaying dissimilar performance characteristics (for example, arrays with different
drive types), the net result is tiering of the data such that the bulk of the active data will typically be serviced
by the array(s) with the most I/O capability. This is shown in Figure 2.
Example of APLB in a tiered environment
The data used to determine what portion of the workload is high I/O is based on recent activity, (on the order
of minutes) so the APLB is able to adapt to a change in an application I/O pattern quickly. The APLB is also
dynamic, constantly evaluating the environment and making small adjustments as required. When an
application has reduced its demand for resources, the APLB does not continue to optimize the formerly active
data. The advantages of the APLB approach are fourfold:
Seamless support of 24/7 business activities: By adjusting incrementally, there are no large batch
movements of data. Instead, the APLB spreads the overhead of rebalancing into small operations through the
day instead of in one large activity.
Ability to adjust to cyclical or one-time workload changes: By evaluating a relatively recent window of
activity, the APLB detects the temporary nature of certain increases in I/O load (such as end-of -month
financial activity), and they do not continue to influence the balancing of data after they are no longer relevant.