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SAS-Lanes and SAS-Expander quesitons (alien.top)

submitted 1 year ago by ITisWeirdSometimes@alien.top to c/homelab@selfhosted.forum

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Hey guys!

I own a HP Server with a p408i-a. I have connected it to a SAS expander.

8 Slots of the SAS Expander are filled with the front disks in the server. 2 Slots are connected to the RAID-Controller. So far so good, everyhing is working. I want to connect another SAS-Device to my expander - will it work?

The p408i-a has 8 SAS lanes. Do I need 1 lane per available/used slot on the expander? Or is it just total bandwidth? Somehow I couldn't find any information about that online. Any help? :)

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[–] cruzaderNO@alien.top 1 points 1 year ago (1 children)

8 lanes just refers that it has 8 lanes directly on card.
That you can connect 2 cables breaking out to 8 ports without the need for an expander.

The expander works similar to a network switch, the cables back to p408i-a is its uplink.
With 2 cables it aggregates the 8lanes to 8x 12gbit and those 96gbit is shared by the devices on the expander.

In the typical dl380 the expander would have 2 cables to each of the 3 front cages + have 2 spare ports for any drives you have in rear of server.
With a maxed out 30x 2.5" front+back server they share the 8 lanes to server.

You can even add another expander to the expander if you want, the card supports 200+ drives if you keep throwing expanders at it.

[–] ITisWeirdSometimes@alien.top 1 points 1 year ago (1 children)

l dl380 the expander would have 2 cables to each of the 3 front cages + have 2 spare ports for any drives you have in rear of server.With a maxed out 30x 2.5" front+back server they share the 8 lanes from expander to server.

You can even add another expander to the expander if you want, the

Thank you, that clears up alot.

So basically I have 3 front cages, each connected with 2 ports, as you said (DL380 Gen10). The expander has 9 ports, so 1 is free to use.

I'd like to connect a tape drive to it. It is using Mini-SAS, while the expander is Slimline-SAS. I found a converter, which uses a PCIe-Bracket (without the port, just passively converts), so from what I understood I should be able to convert Mini-SAS->Slimline-SAS and then just connect it with a slimine->slimline SAS cable to the expander?

How does the speed work - if it is like a switch the bandwidth would only suffer if the Tape is actively using the link? Or will it always decrease because the port is connected?

[–] cruzaderNO@alien.top 1 points 1 year ago (1 children)

You can put a tape drive on the last port like you describe.

Nothing on the expander will suffer unless the full 96gbit link to raid card is saturated and it has to start throtteling stuff.

if you have something 4-8x cache SSDs with high load they would normally go in one cage and that cage gets its own controller.
To not risk saturating the link from expander and impacting performance + avoid the tiny latency expander adds.

[–] ITisWeirdSometimes@alien.top 1 points 1 year ago

Thank you again. I only have 10k 2,5" HDDs, so I guess I won't be close to saturating it. That really helps!

[–] fediverser@alien.top 1 points 1 year ago

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