Test System and Methodology
We will be testing five P400E SSDs on the LSI 9207-8i PCIe 3.0 HBA. This allows for a minimum of limitations placed upon the P400E's during testing. The HBA provides a low latency direct connection to the host system.
We have configured the SSDs as separate volumes to ensure that no RAID calculation overhead will skew the results. Different amounts of overhead and varying levels of scaling with different types/brands of RAID controllers and the controller settings themselves can create fluctuating levels of performance.
We feel that the low latency HBA will provide the clearest view of the characteristics of the SSDs themselves and not the surrounding ecosystem.
Testing Enterprise Solid State Storage (SSS)
When assessing enterprise flash products the parameters are vastly different from the type of testing conducted upon consumer SSDs and PCIe flash devices. With consumer devices the capacity of the SSD isn't always used at 100% fill and the drive is rarely put under a sufficient enough load to drive it down into its worst performance levels.
Flash is a premium tier in professional environments, costing multitudes of order more money per GB than HDD or tape storage. Every penny spent on these EFDs (Enterprise Flash Drives) must be utilized to its fullest potential. This entails using every bit of the storage space available to full capacity and keeping the device under a constant workload for the duration of its lifetime. Unfortunately, this type of high-level usage lines up exactly with the worst case scenario for SSDs in terms of both speed and endurance.
All SSDs rely upon spare NAND (Overprovisioning) to complete the majority of their internal functions, keeping the SSD functioning at the highest levels possible. Spare area also provides a higher endurance over the lifetime of the SSD as well. Since enterprise SSD devices are typically used to full capacity, there are usually extra amounts of overprovisioning that enhance endurance and speed above that of mainstream devices.
After prolonged use, all flash-based devices will start to slow down and reach Steady State. Steady State is the "final' level of performance that the SSD will come to when it is placed under continuous load for an extended period of time. This final level of performance has little variability and is far below the level that is attained when the SSD is brand new and Fresh out of Box (F.O.B.).
Storage Networking Industry Association (SNIA)
SNIA is an industry-accepted group that has defined parameters crucial for assessing enterprise and consumer SSDs. In reality, FOB (Fresh out of Box) test results are rather useless for determining the performance of these devices, so it is critical that we test under the correct considerations. We have adopted several of SNIAs central approaches into our testing program here at TweakTown, which will ensure that we observe the devices under the appropriate Steady State conditions.
Steady State is attained by applying a heavy-write workload over the full span of the SSD over an extended period of time. Steady State can also vary depending upon the type of workload placed upon the SSD, so using the correct type of loading is also important.
The three steps of the process are to apply Workload Independent Preconditioning (WIP), Workload Based Preconditioning (WBP) and then verification of Steady State Convergence.
During the WBP we log the performance data to ensure that Steady State has been attained - the slope is less that 10% min/max. Once we have confirmed Steady State Convergence is within the desirable test range on the graph above, we begin data logging.
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