White Papers

13 Dell Storage PS Series Architecture: Load Balancers | TR1070
Example of APLB in a non-tiered environment
The APLB is surprisingly simple in its concept and execution, leveraging various aspects of the PS Series
architecture to automatically balance the performance delivered to applications by the PS Series SAN. For
example, the rebalance plans that the CLB uses to re-adjust the placement of data, are leveraged by the
APLB as well. Instead of the typical one-way movement that the CLB usually performs, movement of data in
the RBPs that the APLB creates is typically a two-way exchange between PS Series members to ensure that
after a performance rebalance operation, the capacity balance is still maintained.
5
As with all PS Series management tasks, the APLB runs with a lower priority than the processing of
application I/O. Every few minutes, the APLB analyzes the range of latencies of member arrays in a PS
Series pool, and determines if any of the members have a significantly higher latency (20 ms or greater) than
the latency of the lowest latency members(s) in the pool. If it does, the APLB will attempt to identify workloads
that could be rebalanced by moving high I/O data to less heavily loaded members (those with lower latency).
If any are identified, an RBP will be created to exchange a portion of the high-I/O data from the member with
high latency with an equivalent amount of low-I/O data with one of its peers supporting the workload that has
been selected for rebalancing. The peer member chosen for the data exchange will be one of the other
members in the pool already supporting a slice of the volume that has been selected to be rebalanced.
When the APLB is presented with more than one option for rebalancing (the volume selected for rebalancing
has slices on two other members in a larger pool), and the latency of both options is similar, the APLB will use
a second criteria to make the determination. This second criteria is the relative busy-ness of the arrays,
5
In rare cases where a two-way exchange is not possible to achieve, the RBP created by the ALPB creates will still move some of the
hot data to the less busy array. If this creates a low free space condition, then a FSB will be created by the CLB to address the situation.