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drookie
Asked: 2017-01-20 23:56:59 +0800 CST

one expander external enclosure vs one or two cables

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Being given:

  • one external one-expander enclosure 12 Gbit/sec speed per port, two 4-lane ports
  • one 12 Gbit/sec controller with two external 4-lane ports (8e)
  • non-MPIO drives with one interface ports installed in the external enclosure

some engineers state that transfer speed will still be better if one connects the controller and the enclosure with two cables.

Is it an urban legend or a fact ? If the latter, then why (because I don't understand) ? Does it depend on an enclosure model and it's internals ?

hard-drive sas

1 Answers

  1. The speed will indeed increase, when you connect one expander with two cables to one controller you get instead of 2 4-lane connections a single 8-lane connection. The SAS controller and expander will negotiate that between them and it will work just fine. You automatically get more lanes to pass requests and get responses. You do need to more than 4 drives in parallel to get the benefit of the 4 extra lanes.

    In SAS a connection is made on one of the lanes between the controller and the drive through the expander(s). If you have a dual-ported drive and two expanders you can send twice the number of commands in parallel to the drive but that wouldn't help much since the drive (HDD at least) is limited in its ability to provide data, for an SSD there is a marginal benefit for dual-ports. But once you go above 4 drives in the chassis you get a higher benefit just from the ability to communicate with more drives in parallel.

    The way SAS works is that when you send a read request it will open a connection to the drive, send the request (very small) and then close it, when the drive is ready to respond it will open a connection back and send the data. For a write the same happens but the write request will also include the data and the response will be very small. There is actually a fairly high overhead for opening and closing the connections which mostly affects SSDs (10K IOPS vs 100 to 200 IOPS). Having more connections means you can send more requests and get more data back in parallel without having to wait for the contention on the lanes.

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