4k Random Read/Write
We precondition the 400GB SanDisk ULLtraDIMM DDR3 SSD for 15,000 seconds, or four hours, receiving performance reports every second. We plot this data to illustrate the drives' descent into steady state.
This dual-axis chart consists of 30,000 data points, with the IOPS on the left, and the latency on the right. The red dots signify IOPS, and the grey dots are latency measurements during the test. We place latency data in a logarithmic scale to bring it into comparison range. The lines through the data scatter are the average during the test. This type of testing presents standard deviation and maximum/minimum I/O in a visual manner.
Note that the IOPS and Latency figures are nearly mirror images of each other. This illustrates that high-granularity testing gives our readers a good feel for latency distribution by viewing IOPS at one-second intervals. This should be in mind when viewing our test results below. This downward slope of performance only happens during the first few hours of use, and we present precondition results only to confirm steady state convergence.
One of the most impressive benefits of the ULLtraDIMM architecture is reduced latency due to the efficiency of the memory bus. Our normal test suite is multi-threaded, and begins with an OIO (Outstanding I/O) count of eight. As with any normal storage device, once subjected to sustained multi-threaded workloads, the latency will increase. This prevents us from observing the base latency of the ULLtraDIMM. To measure the lowest attainable latency, we conducted a latency test utilizing an industry-standard measurement of 4k random transfers with a single thread at a queue depth of one. Both competing drives, the SATA 6Gb/s Samsung 845DC PRO and the 12GB/s SAS HGST SSD800MH, were tested while attached via an LSI 9300-8i HBA.
A single ULLtraDIMM posted a score much lower than the competing devices, at .011ms. The SATA Samsung 845DC PRO had the highest latency measurement at .036, and the 12Gb/s SAS HGST SSD800MH weighed in with .03. The ULLtraDIMM easily blew away the competition, and SanDisk advises they routinely measure latency as low as .005. There is a distribution of I/O requests that lands below the average results presented during the measurement window. Unfortunately, many test utilities are not well equipped to measure the ULLtraDIMM's extreme low latency. Most utilities simply merge the lowest latency measurements into 00-20ms or 00-50ms buckets. SanDisk specs according to the lowest achievable latency, and IBM has also recorded results as low as .005 with the eXFlash solution (rebranded ULLtraDIMM).
Each level tested includes 300 data points (five minutes of one second reports) to illustrate performance variability. The line for each OIO (Outstanding I/O) count represents the average speed reported during the five-minute interval. 4k random speed measurements are an important metric when comparing drive performance, as the hardest type of file access for any storage solution to master is small-file random. 4k random performance is one of the most sought-after performance specifications, and it's a heavily marketed figure.
With two 400GB ULLtraDIMMs in a RAID 0 configuration at 70% utilization, we averaged 235,254 IOPS at 256 OIO. We also attained similar results at 100% utilization with an average of 233,263 IOPS. With a single ULLtraDIMM at 100% utilization, we received 145,386 IOPS, and with extra overprovisioning, we attained 145,442 IOPS. The benefits of extra overprovisioning typically kicks in during random write workloads, as examined below.
Our Latency vs IOPS charts compare the amount of performance attained from each solution at specific latency measurements. Many applications have specific latency requirements. These charts present relevant metrics in an easy-to-read manner for those familiar with specific application requirements. The results that are lowest, and furthest to the right, exhibit the most desirable latency characteristics.
We include data from a 6Gb/s SATA Samsung 845DC PRO and 12Gb/s SAS HGST SSD800MH in our Latency vs IOPS test. The results highlight the enhanced performance-to-latency ratio attainable via the memory bus with ULLtraDIMM SSDs. One ULLtraDIMM provides increased performance over the competing SSDs, and two ULLtraDIMMs in RAID 0 deliver astounding performance of roughly 240,000 IOPS at the .05ms threshold.
The benefits of extra overprovisioning become apparent in random write workloads, and the two-drive ULLtraDIMM array tops out at 256 OIO with 203,296 IOPS. At 100% utilization, the two-drive array scores 82,451 IOPS. A single ULLtraDIMM at 100% utilization averages 41,289 IOPS, and with 70% utilization, that is boosted up to 106,229 IOPS.
The dual ULLtraDIMMs with 70% utilization deliver extreme performance of 165,000 IOPS at 1ms. The 12Gb/s SAS HGST is incredibly competitive at this workload, but falls behind the RAID 0 ULLtraDIMM configuration.
Our write percentage testing illustrates the varying performance of each solution with mixed workloads. The 100% column to the right is a pure 4k write workload, and 0% represents a pure 4k read workload. Mixed I/O is a constant reality in VDI, and other intensive applications, resulting in the I/O blender effect. The ULLtraDIMM configurations provide impressive mixed workload performance, and recover rapidly from extended workloads. They resist our best efforts to keep them in a steady state during each extended period of the test (from 40-90% mix with 100% utilization), demonstrating their resilience to heavy workloads.
Latency scales within expectations during the mixed workload testing.
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