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I've been tasked with building some large storage and i'm first working to expand my SAS knowledge. As i consider transfer bottlenecks and SAS speeds, i can't help but wonder how 12 Gb/s SAS equates to 4800 MB/s of transfer. From a pure conversion standpoint those numbers don't equate, so i assume it has to do with theoretical vs actual speeds? Or is it just that 12 Gb/s is simply the name of the SAS standard, and this standard is 4800 MB/s of transfer? Thanks for helping me clarify.

ngX55
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  • doing a comment cause not 100% sure but I think the 12Gb/s is the speed from spindle to drive controller – Mike Dec 18 '14 at 20:13
  • Where did you get your numbers from? I thought 12Gb/s was supposed to be good for 9,600MB/s, not 4,800MB/s which is the rate for 6Gb/s SAS. Ref. I am not sure how 12Gb/s relates to the 9,600MB/s value though. – Zoredache Dec 18 '14 at 20:56
  • Mike - you are correct that 9600 is the number that should go with 12 Gbs; i just had that wrong. Thanks. – ngX55 Dec 19 '14 at 14:12
  • I'm pretty sure 12 Gbps = 4,800 MB/s transfers is correct (for SAS-3 at least). https://docs.broadcom.com/doc/12353459 might be helpful. Could it be that it's 12 Gbps per lane? 4x 12 Gbps = 6,000 MB/s minus overhead to arrive at 4,800 MB/S? – Woodgnome May 03 '20 at 14:49

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12Gbps is just an interface speed. Think of it as a link-speed (like Gigabit or 10GbE). Your individual drives will not be capable of 12Gbps transfers. It's just the next step up in the SAS protocol. 3Gbps, 6Gbps, 12Gbps...

This will help with oversubscription in certain external exclosure situations, but without more detail on what you're doing, this is the general idea.

ewwhite
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  • I understand that individual drives cannot push that much data. With the network analogy though, if I had a 1 Gb link i can expect to saturate that link with up to 125 MB/s of transfer. The Gb/s and MB/s numbers represented with SATA transfer don't add up in that same manner of converting Gb/s to MB/s. I can accept it as "that's just the way it is", but i was hoping there would be something more concrete in where the numbers came from. Thanks. – ngX55 Dec 19 '14 at 14:16
  • I'm not sure what's difficult to understand. That is the SAS link speed. But you're limited to what the drive is capable of. – ewwhite Dec 19 '14 at 14:51
  • He's talking about numbers, nothing about drives. There's 8 bits per byte meaning 12 Gbps should equate to 1.5 GB/s, but SAS 12 Gbps transfer rates are defined as 4,800 MB/s. How they arrive at that number is the question. – Woodgnome May 03 '20 at 14:43
  • 4-lane SAS connections. The common transport is a group of 4 SAS connections on say, a SAS SFF-8087 or SAS SFF-8643 connection. – ewwhite May 03 '20 at 18:06
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I know this is a really old question at this point, but I see that it is still open, and I went down a similar rabbit-hole trying to optimize a spreadsheet I was creating to monitor disk failure. Long story short, it has to do with how the data is encoded. The most common encoding scheme is 8b/10b, but there are a multitude of them out there (https://en.wikipedia.org/wiki/8b/10b_encoding) and what this means, is that you are essentially taking the stated speed (in this case 12 GB/s or 12,000 MB/s) multiplying by 8 and dividing by 10. This gives you the 9,600 MB/s. It is obviously way more complex than this (read the articles on encoding, it will seriously make your head spin), but this is the simplest explanation I could find to at least allow me to get some sleep at night.

Edgar
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