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SAS vs SATA Confusion
Mahfuz_SS_EHL
Host Rep, Veteran
in Help
Hello,
I was analyzing 2 types of chassis. One having 4 SATA Ports in Backplate (Hot-Swappable) another having 4 SAS Ports in Backplate (Hot-Swappable).
All the recent motherboards have 6Gb/s individually on each port ? If I read/write data concurrently on 4 drives, will each get 6Gb/s ?
The SAS chassis has a MiniSAS Interface (SFF-8643) & needs a converter to connect to SATA Drives. But, the cable is 12Gb/s. Then, 4 of My drives will get cumulatively 12Gb/s, isn't it ??
BackPlate:
Details:
Converter:
Comments
You can put SATA drives into SAS backplane slots. No converter needed.
As for speed, no matter the interface, there is a theoretical max speed (6 Gb/s, 12 Gb/s, ...) but how much you get out a set of drives (e.g. 4 on a backplane) depends on diverse technical details like the connectivity between controller and backplane, extender, controller-PCIe interface and speed.
Note that the whole speed question is a rather theoretical one anyway unless you use hellishly fast SSDs or NVMes. It's worth remembering that HDDs are what the whole shebang was originally designed for.
So example: 3 HDDs with each 200 MB/s wont be a problem even if they (just for the sake of explaining) all hung on a single Sata-3 interface (e.g. using a cheap extender).
Crude rule: look at the controllers interface (e.g. 4 PCIe lanes vs 8 lanes) and don't worry about 6 or 12 Gb/s with HDDs. In most boxes I looked into the slowest element of the chain was always the HDDs and, nowadays, NVMes being connected via SAS because people want, want, want NVMes even in older chassis. (I'd advise to use a PCIe interface "carrier" on which one can mount one or two M2 NVMes).
You mean I can put My Drives straight in the Hot-Swap bays, right ?? But, what about if I connect 4 SSDs ?? SSDs not only theoretically but practically push to 550MB/s easily. At that time, won't it bottleneck for the 12Gb/s Capacity ? And, isn't it possible to connect the SAS Backplane directly to SATA Ports of Motherboard ? Why would I need the SFF-8643 ?
Yes.
I think that's largely BS. The meanwhile quite many benchmarks I've run on many and diverse systems have shown that "550 MB/s" as well as often cited SSD specs are ideal case numbers and not sustained real-world values.
No, because that is the max speed between controller and drive. A controller linked via PCIe x 8 can easily carry the data of multiple 6 Gb/s or even 12 Gb/s devices. The potential true slow down could come from an inadequate port extender, but even that is unlikely with SSDs.
You are confusing different things like ports, protocols, and connectors. If a controller offers SAS ports one can connect SAS as well as SATA drives. If however it would offer SATA ports only SATA devices could be connected.
Also note that there are controller-backplane combos using neither SAS nor SATA but PCIe or other protocols. This is particularly common with modern systems which also support NVMe drives.
Yes, I realized now, I have confused these things.