The newer 12gbps LSI cards and expanders are supposed to do this. I believe the 12gbps PMC Sierra expanders will do this too. I have an Intel expander sitting in the lab with no time to test unfortunately.
The LSI technology is called DataBolt IIRC.
It looks like the LSI DataBolt (you recalled correctly) require a 12G card and 12G expander to give all of the performance benefits to 6G SAS and SATA drives. It's a great idea but it only looks like one or two SC846s have the 12G backplane, and I doubt I can find them for a cheap price.
Between these two following discussions I think I got the answer to the two questions I was pondering:
Bought a used server, need some background on the backplane
https://forums.servethehome.com/index.php?threads/need-help-with-home-storage-server.4237/
(PigLover seems pretty smart - good thing they are a moderator here
)
The first is that technically breaking out each SAS port on an HBA to 4 channels to individual drives, would net a better performance because there would be no chance for oversubscription (what I was concerned about with accessing 20 drives through the single 24Gb/s 4 x wide SAS channel) and better latency. However this approach is limited in the amount of drives that are supported to being 4 per port * the number of ports on the HBA.
The second is that dual connecting a SAS HBA to a dual ported expander would do nothing for SATA drives as they are single channel. I would need interposers either in the expander or with each drive, something that's not going to happen with one of the SC846's and 3.5" HDDs. I think I even read that trying to connect a single HBA to both ports on the SAS2-846EL2 would cause issues with SATA drives.
So I can go with one of the SAS expander backplanes (the single port model would be fine for SATA drives) supporting all the drives I want over a single 24Gb/s cable/wide channel, or get a non-expander backplane and break out each drive individual with the limitation being the number of drive ports I can support.
The reality is I will probably never come close to hitting 24 Gb/s AKA 3000MB/s except during a local (non-network) stress test. So in theory the single SAS2 expander connection should be sufficient, of course unless the 24Gb/s isn't shared efficiently between 20 drives, like it is split into 4 lanes that have to be rotated to 4 drives at a time (yes I am overthinking things again, I tend to do that
).
At this point I think I will go with the SAS2 expander unless they aren't available at a reasonable price compared to the non-expander. The additional benefit with this approach is I only have to buy a dual-port HBA versus a quad port, because I will only be using one internally and will have one for future expansion.
Thank you everyone for your feedback, information sharing, and overall patience as I came (relatively) up to speed. If anyone sees anything wrong with what I wrote or has a different POV, please share it.
