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9 Dell PS Series Storage: Choosing a Member RAID Policy | TR1020 | v 4.7
2.2 RAID policy impact on performance
A PS Series group provides storage to a broad range of environments and delivers good performance for a
variety of workloads and applications, regardless of the RAID level on the member. However, for some
applications, choosing the correct RAID level can make a difference in performance, under both normal
operating conditions and failure conditions.
Determining which RAID level is best for an application depends on the following factors:
 Workload (heavy or light)
 How the application performs I/O (small random transfers or large sequential transfers)
 Read/write ratio
 Impact of degraded and reconstructing state
For example, video editing, streaming media, and disk-to-disk backup applications mainly perform large
sequential transfers. Database and mail applications perform small random transfers. However, if multiple
applications are accessing the storage, the total load may become random in nature, even if individual
transfers are sequential.
An important tool in assessing performance of different RAID policies is a suite of benchmarks called “Four
Corner” testing. The graphic below illustrates the four combinations of I/O characteristics highlighted above:
 100% sequential writes
 100% sequential reads
 100% random writes
 100% random reads
Note: The graphic below illustrates four corner testing results as compared to (or normalized to) RAID 50
performance for each corner. This graphic does not illustrate the differences between corners (such as
random write versus sequential read), but rather just compares RAID performance within a corner (RAID 6
versus RAID 10 for random read). Absolute performance comparisons are discussed later.
While there are modest differences between RAID policies within a corner (aside from random writes), there
can be significant differences in the absolute IOPS performance when comparing between corners, that is,
random write to sequential read.