[SOLVED] How many drives on a single HBA or SFF 8087 port using SAS expanders before individual hard drives cannot reach their maximum speeds?

sds20020024

Reputable
Jan 23, 2019
50
8
4,565
10
Idk if this belongs here but here goes.

Sorry if this question is silly. I'm kind of a networking and storage noob.
So I recently got interested in storage servers and learned about SAS expanders and stuff.
I didn't know that SAS expanders were a thing, and I thought that if you wanted to connect 24 drives, you needed an HBA with 3 SFF8087 (one breaks out to 4 sata cables) connectors.
But now I've been looking at 60 bay disk shelfs like this, and they usually have QSFP+ or sff8087 ports on the back, and use SAS expanders.
I've learned that SAS expanders can be thought of like network switches, and they can distribute the bandwidth.
My question is this: let's say I got one of those 60 bay disk shelves, and ran one mini sas cable to my HBA on my server. Would individual drives be bottlenecked?
When an HBA like this is 6Gbps, does it mean that all the SAS ports are 6Gbps each or is the entire card running at 6Gbps?
Is the total bandwidth of the card the total bandwith of x8 pcie slot?
Seagate Exos enterprise 16TB drives advertise a max transfer rate of 260 MBps. if each mini sas sff 8087 cable has a max theoritical bandwidth of 6Gbps, can it run about 3 of these drives and adding more drives to one cable would mean each drive cannot perform at it's max?
This 60 bay disk shelf, for example, has 8 (looks like QSFP+ , idk I've never seen one in person) ports, so should I get two HBAs with 4 ports each so that I can connect to all of those 8 ports?
Would connecting to just 4, for example starve the individual drives of bandwidth and they won't be able to perform to their max speeds?
How about connecting just one cable?
I'm trying to find out whether it's just simple match, i.e, drive max speeds, and sas cable max bandwidth, and you don't run more drives off of one cable when the individual drives get bandwidth limited.
 

RealBeast

Titan
Moderator
It depends on the capabilities of your SAS controller card. Many support up to 128 devices, but some support 256 devices. I strongly encourage you to insure that all your parts are compatible before buying anything.

And you shouldn't really look at the transfer limit with the maximum speed of a single drive, as that is an optimal sequential transfer rate. Much depends on how you use the drives, JBOD or some RAID level for example.

Here is some basic information that may answer a lot of your questions at this early stage.
 

RealBeast

Titan
Moderator
It depends on the capabilities of your SAS controller card. Many support up to 128 devices, but some support 256 devices. I strongly encourage you to insure that all your parts are compatible before buying anything.

And you shouldn't really look at the transfer limit with the maximum speed of a single drive, as that is an optimal sequential transfer rate. Much depends on how you use the drives, JBOD or some RAID level for example.

Here is some basic information that may answer a lot of your questions at this early stage.
 

ASK THE COMMUNITY