SAS3 to dual SAS2 breakout

Discussion in 'RAID Controllers and Host Bus Adapters' started by capn_pineapple, Feb 4, 2015.

  1. capn_pineapple

    capn_pineapple Active Member

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    So today, one of the junior helpdesk guys asked me if it was possible to go from a single SAS3 port and cable it to two different SAS2 expanders.
    His situation is that he snagged a cheap 4i SAS3 board off ebay and already has two SAS2 12i expanders (1x in 2x out on each card). He only has spinners, no SSD's and certainly nothing that will take the full bandwidth of the sas3 port. I'm guessing he wants to split the bandwidth over the two expanders.

    My immediate reaction was "no" because I don't know of any breakout cables to do what he wants, and that he should sell his 4i SAS3 card and get a 8i one. I thought I'd double check though.
     
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  2. Chuckleb

    Chuckleb Moderator

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  3. Patrick

    Patrick Administrator
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    That is what I would do in that situation.
     
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  4. capn_pineapple

    capn_pineapple Active Member

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    He's decided to use the SAS3 card for some SSD's and grab an M1015 off ebay

    Actually, this brings up a question from me. If he got a SAS3 expander, as in a single input to two outputs, with those two outputs going to SAS2 expanders could the single 12Gb/s "trunk" be saturated if divided between two 6Gb/s expanders then populated by enough SSD's to saturate the 6Gb/s links? Similar to a tree/leaf network.

    I believe that's how single hba's can address "hundreds" of expander drives.

    I think I just answered my own question... Or is there a significant hit to performance in doing the above?
     
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    Last edited: Feb 4, 2015
  5. dba

    dba Moderator

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    You can't "split" your SAS3 into two SAS2 connections, but you can daisy chain them. You would connect the HBA/RAID card to the "in" port of one expander, then connect the "out" port of that expander to the "in" port of the second expander. You would need one SAS3 to SAS2 cable like the one Chuckleb pointed to and one SAS2 to SAS2 cable.
     
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  6. capn_pineapple

    capn_pineapple Active Member

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    Does the full chain then perform at SAS2 speeds? i.e. weakest link
     
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  7. Chuckleb

    Chuckleb Moderator

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    I've never tried, but I would think so. The SAS2 cards are rated 6Gb/s links. You'd need to run it into a SAS3 expander... so your bill just shot through the roof. If he's not doing SSDs, then the SAS2 daisy chain method should be more than fine. Still cheaper than upgrading to an 8i probably.

    IntelĀ® RAID Expander RES3FV288

    So now it goes:
    SAS3-4i card -> SAS3 expander -> SAS2 expanders

    ;)
     
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  8. Chuckleb

    Chuckleb Moderator

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  9. capn_pineapple

    capn_pineapple Active Member

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  10. dba

    dba Moderator

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    Yes it does.
     
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  11. TuxDude

    TuxDude Well-Known Member

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    I thought that problem was fixed with SAS3. A SAS2 expander has to function at the speed of the slowest device, but a SAS3 expander can do 12G speed to the HBA while mixing 3/6/12G speeds across devices. Though even that doesn't quite align with what I see from my own setup at home - my IBM 1015 says it is running its links at 6G though I do still have a few 3G SATA drives connected to the expander.

    And back to the question in the OP - I believe it actually should be possible to do, and should work for SAS1, SAS2, or SAS3 controllers. You should be able to break out the SFF 8087 connector (or SFF-8643 or any other quad-SAS cable) into its 4 individual SAS lanes - a common thing to do to direct-attach drives to a controller with quad-channel ports. Then with reverse-breakout cables, connect 2 of the lanes to one expander, and 2 lanes to the other expander. I wouldn't recommend doing it (I would just daisy-chain expanders), but in theory it should work.
     
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  12. dba

    dba Moderator

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    Yes, I do believe that the mixed drive speed problem is fixed with at least the LSI SAS3 expanders, but I don't think that helps in capn_pineapple's case. With the single port SAS3 HBA (4i as described by capn_pineapple), a SAS2 expander connected to that HBA, and another SAS2 expander connected to the first HBA, the best you'll get is x4 SAS2 throughput, which is around 2GB/s. For most users, however, that's should be plenty - enough to fill two 10Gbe network ports.
    You mentioned splitting the x4 to four x1 with a splitter cable and then combining two x1 to an x2 twice with two combiner cables. It might work (though I've never seen an x2 wide port myself) but as you point out, a simple daisy chain is going to be simpler, cheaper, and just as fast.
     
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  13. TuxDude

    TuxDude Well-Known Member

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    I've never seen one either, and am fairly certain such a thing does not exist. But nothing prevents you from leaving 1/2 of the connectors on each of the combiner cables disconnected, only using 2 of the 4 lanes in the SFF-8087 connector that ends up plugged into the expander. If you have a damaged cable or connector on a regular SFF-8087 cable you can easily end up with only 3 of 4 lanes functional - what I described above is kind of like a 'planned failure' of 1/2 of the lanes in each combiner cable.
     
    #13
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