Ok, actually, my questioning is a bit more specific. Let's take an example of the HP DL320s: http://h18004.www1.hp.com/products/quic ... 71_na.html
It supports 14 SAS/SATA drives. Now the integrated controller is a P400: http://h18000.www1.hp.com/products/quic ... 00_na.html
Now here's where I get a bit lost. The P400 has "Eight (8) SAS physical links distributed across 2 internal x4 wide port connectors." My understanding of SAS is that that would be 8 SAS/SATA drives. However, "The SA-P400 Controller supports up to 18 drives depending on the server implementation."
...what? And even more: "Up to 38TB of total storage with 38 x 1TB 3.5" SATA MDL HDD"
So it can support 8 drives? Or 18? Or 38? But all with 8 physical links/channels for SAS?
Or how about the P800: http://h18004.www1.hp.com/products/serv ... index.htmlhttp://h18000.www1.hp.com/products/quic ... 16_na.HTML
You can put a P800 in that DL320s to handle the internal drives and hook up to an external SAS enclosure. 16 physical links via 2 x 4x internal and 2 x 4x external SAS connectors. Now this sucker can link to an external SAS chassis like thus: "Up to 96 TB of external storage per PCI slot with 8 HP 60 Modular Smart Array enclosures and 96 x 1TB 3.5" SATA MDL hard drives"
The MSA 60 enclosure is this: http://h18004.www1.hp.com/products/quic ... 7_div.HTML
I get that you can daisy chain a bunch of them. But I still have issues with the physical SAS links.
Do the SAS backplanes built into these servers and enclosures multiplex all the drives and just flood the SAS interface going to the controller with the max bandwidth it'll take? Like an ethernet switch, conceptually? 7 ports on your ethernet switch are all dumping max data and it's getting parceled out into the one remaining output port, so to speak?
I see some DIY storage solutions where you need, say, 20 drives so you have 1 controller with 5 SAS 4-channel ports or multiple SAS controllers to handle all the drives. Yet HP and other vendors like Dell have controllers that seem unable to handle the physical load of all the drives they claim to handle. What do they do different?