Question Sas bandwidth from PCIE considering bidirectional nature?!

[FreeBee101]

I'm looking at sas drives setups for a theoretical 192TB 8x2(16drives) raid 10 setup. I'm trying to figure out the max I can use in a 16xPCIE 3.0 motherboard for the new NZXT 870 board. I'm ignoring the potential heat problems atm to see how much it can do.

What I'm confused on is the PCIE bandwith. I assume the cards are bidirectional at the PCIE card lane and hence a PCIE 3.0 should be able to do 32GB assuming the card is 16x and not 8x. If it's 16x does that mean the most you could do is a SAS3 card with 12gb/s per card for 192gb read and 96gb write?

Is this in parity with what the PCIE slot has to do? Are the drives 12gbps one way or is it 6gbpsx2 with bidirections. I'm assuming the latter as I was assuming it's the same as SATA 6gbps but duplexed. Is bidirectional 6gbps the max you want to get as 12 would be 1.5GBps as pcie 3.0 can only do 1GBps per lane directionally? Or does the card do something to the data that makes that not matter?

Would there be a way to get 24gbps sas4 drives in such a setup. Could you do a 4x2 48TB ssd version of the cards on a 3.0pcie if the total bandwidth is lower over the raid?

I thought I saw there were SAS4 SSD drives with up to 4300MBps random Sequential read/writes or something. Are those better than the old HDD version. I would rather have smaller drives theoretically.

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Its sequential read speed can reach 4,300MB/s, which is the maximum available speed for the 24G SAS interface and twice the speed of the previous-generation PM1643a drive.

This is all theoretical. Not considering other issues.

 

[BFG-9000]

Technically, SAS is full-duplex unlike SATA, so SAS-4 can simultaneously read and write, both at 22.5 Gbit/s ("24G") at the same time with some caveats such as this not really being usable except at extended queue depths.

Given that 45Gbit/s is only 5.625GB/s, PCIe 2.0 x16, PCIe 3.0 x8, PCIe 4.0 x4 or PCI 5.0 x2 should be plenty to supply that, and even halving those should not appreciably slow a single drive. I'm not sure how they can claim 4,300MB/s sequential though when 22.5Gbit/s is already under 2.9GB/s before overhead (which is the reason why say, 6Gbit/s SATA-III tops out around 560MB/s).

BTW, twice-as-fast SAS-5 which has been in development since 2018 is supposed to drop this year, so that old SAS-4 drive from 2021 will soon look like old hat. But for most people loading up as many NVMe drives as will fit would be a better option.