Understanding SAS Link throughput and saturation.

[m-a-x]

I've read some older posts here and came across a statement that left me confused. I need your help confirming or disproving it.

 

Assuming we have a 24 slot SAS expander backplane supporting 12G and an HBA card supporting 12G.

We connect them using one 4-lane Mini-SAS cable, effectively yielding us a bandwidth of 48Gbit/s.

 

The statement that I came across says that if we plug, say 12 x SATA300 drives, the speed will be limited to 4 (Mini-SAS lanes) x 300MB (drive speed) = 1200MB/s. TRUE or FALSE?

 

If TRUE, then please explain what is preventing the SAS expander to push 12 x 3gbit = 36Gbit/s over the 48G link?

 

[JorgeB]

It was true with SAS2 expanders and HBAs, and AFAIK there are no exceptions, with SAS3 there are, LSI for example uses Databolt to improve performance with slower linking SAS2/SATA3 devices:

 

 

PMC Sierra uses a similar feature, some numbers in the thread below:

 

https://forums.unraid.net/topic/41340-satasas-controllers-tested-real-world-max-throughput-during-parity-check/

 

[m-a-x]

9 hours ago, JorgeB said:

It was true with SAS2 expanders and HBAs, and AFAIK there are no exceptions, with SAS3 there are, LSI for example uses Databolt to improve performance with slower linking SAS2/SATA3 devices:

 

 

PMC Sierra uses a similar feature, some numbers in the thread below:

 

https://forums.unraid.net/topic/41340-satasas-controllers-tested-real-world-max-throughput-during-parity-check/

 

 

My perception might be completely off, due to some things that I could be missing entirely. But if we forget about SAS3 and DataBolt for a moment (I found this info too while waiting for comments here) and consider an older SAS2 end-to-end connection but SATA2/SAS1 drives - what actually happens there?

 

I just can't understand how the speed of the drive can reduce the bandwidth of the link between the expander and HBA.

Which hardware instance decides to drop the speeds to 3Gbps per lane when it detects SATA2 drives?

What happens if I have a mix of SATA2 HDD and SATA3 SSD capable of 6Gbps?

 

 

[JorgeB]

I know it happens, can't tell you exactly why as it's beyond my knowledge, likely because an expander is kind of like a switch.

 

12 hours ago, m-a-x said:

What happens if I have a mix of SATA2 HDD and SATA3 SSD capable of 6Gbps?

It will downgrade the performance of all connected devices, there are a couple of examples of that on the link I posted above.