6 GB SAS over PCI-e 1.0??

[flingom]

Hi,

I'm trying to understand whether there are any products that support 6 GB SAS back-end using PCI-e 1.0. It looks like all the 6 GB SAS products require PCI-e 2.0 to get maximum throughput. My understanding is that PCI-e 2.0 doubles the throughput, so if a PCI-e 2.0 could support 2 x 6 GB SAS ports, why couldn't PCI-e 1.0 support 1 x 6 GB SAS port?

Thanks a lot for the help!!

-John

 

[ViviTheMage]

Capacity Per lane:

* v1.x: 250 MB/s
* v2.0: 500 MB/s
* v3.0: 1 GB/s

16 lane slot:

* v1.x: 4 GB/s
* v2.0: 8 GB/s
* v3.0: 16 GB/s

I do not think that PCIe 1.0 can handle that at full throughput ?

EDIT : Read this, it may be a little more informative :

http://zone.ni.com/devzone/cda/pub/p/id/739

They say it's capped at 4GB/s, per direction.

You are right though .. PCIe 1 should be 8GB/s , and PCIe 2.0 should be 16GB/s if the math is correct.

 

[flingom]

Thanks for the quick reply. However, those data rates are, I believe, in megaBYTES. The SAS data rate is in gigaBITs.... So 250 MB/s is actually 2 Gb/s... So 8 channels of PCI-e 1.0 should be able to support 16 Gb/s, more than enough for the SAS data, right??

 

[MerlinRML]

All numbers are theoretical maximums. Capital B is bytes, lowercase b is bits.

PCIe 1.0 = 250MB/sec per lane. A 8x PCIe 1.0 card can handle 2GB/sec. 6Gb SAS is 750MB/sec per port. Connect 2 drives and you're fine with 1500MB/sec SAS bandwidth and 2000MB/sec PCIe bandwidth. If you're using a 4x PCIe 1.0 card, then you've got a mistmatch between the SAS and PCIe buses creating a potential bottleneck.

Realistically, mechanical drives can't push over 200MB/sec, and that's peak performance under perfect conditions. So the reality is that none of this really matters unless you're hooking up a lot more disks or have an expander, or you've got some early samples of 6Gb SAS SSDs that can burst up to infinity.

 

[pitz]

Mechanical HDD's rarely push out more than a few MB/second unless its simply linear I/O.

So don't sweat it!