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SAS expander speed - bandwidth

J

Jeroen1000

Limp Gawd
Joined
Sep 17, 2010
Messages
266
Hello everyone,

I've took my time to research this and although it does not apply to my setup directly, I'm quite curious to know. The answer is probably hidden in the HP SAS expander thread...

Here goes:

General facts:
Areca HBA ---- SAS Expander (3 Gigabit/s) --- 20 drives
Sata tunneling protocol limited to 800 megabyte/s.

Factoring in encoding overhead:
(3 gigabit) * 8 / 10 = 2.4 gigabit/s.

So, is the connection between the Areca and the expander 2.4 gigabit times 4 (yielding 9.6 gigabit/s)? Because the SFF-8087 connector has 4 lanes. Or..not:)?

Thanks for the clearing this up.
 
Last edited:
I was quite sure about it being gigabit and not gigabyte.

http://en.wikipedia.org/wiki/Serial_attached_SCSI. The right hand table says
3.0 Gbit/s at introduction, 6.0 Gbit/s available February 2009, 12.0 Gbit/s in development

So 2.4 gigabit/s is about 300 megabyte/s. So it would make sense (as 300 megabyte per second is very little for potentially 32 drives on the HP expander) to do this number times 4 yielding 1200 megabyte/s.
 
Each multilane cable consists of 4 lanes (some 8 lane cables exist, but I think only 3ware used them briefly), so you can essentially quadruple your figures. Keep in mind that you can also run multiple cables to get 8 lanes of throughput total. The only thing that will be limiting you is going to be the processor on the RAID HBA.
 
so 1200 megabyte per second is the correct figure as 1 lane gives you about 300 megabyte per second ( =2.4 gigabit)? Anyway, this would only matter when a rebuild happens. As I've just bougth 5, 2 TiB drives I hope the rebuild time to expand the RAID6 array is acceptable.
 
Yes, that's what you're looking at using a 3gbit HBA and/or expander and a single multilane cable.
 
Thanks. Now I can place the numbers I get with benchmark tools. Btw:) I hope all is well there with you accross the pond!
 
Doing alright here. Just don't forget that you're going to be limited by the RAID HBA and not the SAS expander, so if you're doing any benchmarks it's unlikely that you'll get much of a performance gain from using two cables over just a single one.
 
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