White Papers

12 Dell Storage PS Series Architecture: Load Balancers | TR1070
7 How the Automatic Performance Load Balancer (APLB)
works
The APLB feature is designed to help alleviate the difficulties inherent in manually balancing the utilization of
SAN performance resources. Operating on the resources in a pool, The APLB is capable of adjusting to
dynamic workloads in real time and at a sub-volume level. It will provide both sub-volume-based tiering when
presented with heterogeneous or tiered resources to work with, as well as hot-spot elimination when
presented with homogeneous resources in the pool.
The APLB optimizes resources in a PS Series pool based on how the applications are actually using the SAN
resources. When the slices have been assigned to members in the PS Series pool by the CLB and I/O
begins, certain patterns of access may develop.
3
Due to the random nature of I/O, these access patterns are
often unbalanced, which while normal, may place more demand on certain PS Series members than others.
Often, the imbalance occurs within the same volume with portions of the volume exhibiting high I/O, while
other portions of the volume exhibit low I/O. This imbalance can be detected and corrected by the APLB.
In a PS Series pool, all other PS Series products can adjust to this potential imbalance in latency. In the event
that a workload causes a particular PS Series member to exhibit relatively high latencies compared to other
members of the same pool, the APLB will be able to detect and correct this imbalance and by exchanging
high I/O data from the PS Series member with high latency for low I/O data from a peer with low latency. This
rebalancing results in better resource utilization and an overall improvement in the performance of all of the
applications using the resource of the PS Series pool.
4
Figure 1 shows a basic example of this across two
members.
3
The response time required to service I/O requests to and from applications is referred to as latency. The I/O requests can be either
random (access non-contiguous data) or sequential, where the data that is accessed in contiguous. Sequential data access is often
exhibited by applications such as streaming media, and may be more tolerant of higher latency response from the storage as long as the
stream of data is consistent, while random access is often less forgiving and requires lower latency when users are actively using an
application in order to provide an acceptable user experience.
4
Classifying data as high or low I/O is determined by the frequency of access to the data by the applications over a relatively brief
interval.