SAS expander backplane performance effects

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A SAS expander backplane enables the connection of more storage media to an HBA or RAID controller than these controllers themselves natively support. In this way, for example, 24 SSDs can be connected to an 8-port HBA or RAID controller. The SAS expander Chip is located in the I/O path and influences the maximum possible performance. In this article we show the performance effects of the SAS Expander on latency, IOPS and data throughput using the example of an Intel D3-S4610 Series SSD.

I/O When using a SAS-Expander backplane, accesses naturally run through the SAS Expander chip. This increases the latency and influences IOPS and throughput. In return, it allows more data carriers (e.g. 24 instead of 8) to be connected to an 8-port controller.

Example setup

The following example setup was used in this test:

Component Onboard test Expander backplane test
Motherboard Supermicro X11DPi-NT with BIOS 3.0c
Test-SSD Intel D3-S4610 960 GB with FW XCV10110
SSD connected via onboard via C622 Chipset HBA and expander backplane
HBA - Broadcom 9300-8i with FW 16
SAS expander backplane - BPN-SAS3-216EL1

Test results

The test was performed with a single SSD to analyze the effects of the SAS Expander on latency, IOPS and data throughput. Information on the effects of using multiple SSDs (e.g. 24 pieces) is documented in the article I/O data throughput limits.

The following four graphs show the differences in performance:

  • IOPS test: With 100%Read and 50%Read/50%Write approximately the same performance, with 100%Write approx. 15.6% less IOPS via HBA+Expander.
    IOPS test: With 100%Read and 50%Read/50%Write approximately the same performance, with 100%Write approx. 15.6% less IOPS via HBA+Expander.
  • Latency test: Depending on the test case, HBA+Expander increases latency by 4% (100% Read) to 12% (100% Write).
    Latency test: Depending on the test case, HBA+Expander increases latency by 4% (100% Read) to 12% (100% Write).
  • Throughput test: When reading, noticeable differences only occur with smaller block sizes (8k); when writing, the maximum throughput through HBA+Expander is reduced by 15.3% (with 1024k) to 18.6% (with 4k).
    Throughput test: When reading, noticeable differences only occur with smaller block sizes (8k); when writing, the maximum throughput through HBA+Expander is reduced by 15.3% (with 1024k) to 18.6% (with 4k).
  • Write saturation test: As with the IOPS 100%Write test, the number of IOPS is reduced by approx. 15% when using HBA+Expander.
    Write saturation test: As with the IOPS 100%Write test, the number of IOPS is reduced by approx. 15% when using HBA+Expander.

Test protocols in detail:

Conclusion

Depending on whether the highest possible performance or the highest possible number of storage media is required, it makes sense to use a server with a discrete backplane or SAS expander backplane:

Demand Recommendation
Highest performance Server with discrete backplane, systems with up to 12 storage media slots possible,
e.g. Product not available anymore
High number (>12) of storage media Servers with an expander backplane offer up to 36 disk slots,
z.B. Product not available anymore

Author: Werner Fischer

Werner Fischer, working in the Knowledge Transfer team at Thomas-Krenn, completed his studies of Computer and Media Security at FH Hagenberg in Austria. He is a regular speaker at many conferences like LinuxTag, OSMC, OSDC, LinuxCon, and author for various IT magazines. In his spare time he enjoys playing the piano and training for a good result at the annual Linz marathon relay.


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