SAS expander and RAID 10 throughput confusion

Discussion in 'RAID Controllers and Host Bus Adapters' started by s3ntro, Jul 8, 2018.

  1. s3ntro

    s3ntro Member

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    Hi all, I'm hoping you can help me understand the throughput of my intended storage build.

    I'd like to get 12 SATA HDDs in a RAID 10 configuration to get closer to 10gb write. This will be on a consumer motherboard but with an Intel SAS 3 controller (Intel® RAID Controller RS3DC080 77346). One question I'm struggling with is whether to go with a SAS2 or SAS 3 expander, which would be connected directly to the hard drives via breakout cables.

    Here's what I'm thinking: the spinning hard drives probably won't hit 187.5 MBps each but if they did, each group of 4 hard drives would hit 6 gbps. In RAID 10, this means a real/unique data throughput of 3 gbps (due to mirroring). So, I need 12 hard drives to get 9 gbps.

    Assuming I aquire a SAS2 expander, I will have two 6 gb links from the RAID card to the expander. Here's where my question about unique data throughput comes in. If the RAID controller has to do the mirroring then half of the 12 gbps throughput to the expander card is wasted. If the expander can handle the mirroring then the full 12gbps of the link can potentially be used for unique data. I have no idea if this is how it works. Please tell me more!
     
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  2. Aestr

    Aestr Active Member

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    Each cable from your RAID card to your SAS expander will carry 4 lanes, so if your expander is SAS2 you'll get 24gbps per cable. If your goal is only to saturate 10gb the bandwidth between your expander and the controller will not hold you back at SAS2.
     
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    i386 and s3ntro like this.
  3. s3ntro

    s3ntro Member

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    Oh I thought the ports themselves were limited to 6gb (not each link). Thank you for the clarification.
     
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  4. i386

    i386 Well-Known Member

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    1x sff-8087/8643 port = 4x sas ports (compatible with sata)
     
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    Tha_14 likes this.
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