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18 Dell Storage PS Series Architecture: Load Balancers | TR1070
10 Using the APLB with hybrid arrays
The PS Series hybrid arrays (such as the PS4210XS, PS6210XS, and PS6610ES) use their own internal
implementation of the APLB to compensate for unbalanced or tiered workloads. These arrays shift the active
parts of the workload in the volume slices that they are responsible for to SSD media for extremely low-
latency I/O, while leaving the bulk of the workload on HDD media for normal latency I/O. This lowers the
overall latency of the hybrid products for the workloads that they are supporting.
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Since the CLB treats hybrid arrays like any other member in the PS Series Group, capacity will be assigned to
them in relation to their overall capacity contribution to the pool. The APLB is able to work in conjunction with
the hybrid arrays, permitting other, non-hybrid arrays to migrate active data to the hybrids in exchange for
inactive data. In the case where more active data is placed on a hybrid array than the SSD tier is able to hold,
the APLB is able to migrate active data from the HDD tier to another member such as a second hybrid or
standard array that may have more resources available to better service the I/O requests. An example of a
pool containing both a hybrid array and a non-hybrid array is shown in Figure 3. An example of two hybrid
arrays in the same pool is shown in appendix A.
Additional details on APLB can be found in the document, EqualLogic PS Series Architecture: Hybrid Array
Load Balancer.
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The hybrid arrays use only access frequency, not latency, when deciding what data to optimize internally. This, and the lower overhead
of moving data internally, permit the hybrid arrays to optimize their workloads internally at a higher rate than the inter-array optimization
of the APLB within the pool.