Intel 36-port SAS3 expander - Really only 24 SATA HDDs directly connected possible?


I’m in the process of building a custom disk shelf solution and something that popped up is that the 36-port SAS 3 expanders I have been using for a few years can only support 24 directly attached HDDs?! (By chance this was the number of drives I had been using for about two years so I never exceeded this limit until now).

I purchased them with the future plan to use up to 32 directly attached drives - but (changed?) Intel documentation says something about being only able to use 6 of the SAS x4 ports for drives and 3 x4 ports are only usable to connect to HBAs?

Is this correct? There is a thread mentioning this in the ServeTheHome forum but “real details” remain “unsolved”. I’ve always thought that SAS expanders act like network switches and they don’t care what number of drives you connect to them as long as you don’t reach like >100 by connecting several expanders as a daisy chain.

Does anybody here know the technical reasons for this?

There is a limit, which should be listed on the HBA/Raid card’s spec (iirc, some do like ~ 25 or ~250 devices per SAS socket, with expanders, but SAS drives can consume two per drive) but at the end of the day, the hard limit would be the pci lanes, as absolute max

for example of daisy chaining

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Yes, this had been my understanding as well: Your HBA limits the maximum drive/device count or it will get so slow that it doesn’t make sense to add more drives (but I don’t think this is the case with 24 spinning SATA drives).

But somehow the Intel SAS3 expander seems to have a limit of its own… :frowning:

I’ll try to find the Intel pdfs where this sh** is stated.

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There you go, from Intel’s hardware guide:

Tested a RAID controller with it (LSI/Broadcom MegaRAID 9361-4i) and it’s management software states that the expander itself has only 28 slots in total available.

PS: All devices have the latest firmware installed (non-branded retail components).

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No biggie, and also, the more devices, the less bandwidth and cache each can use.

And even 28 slots is more than the 24 you need?

I previously only used 24 drives directly attached - now I’d like to go to 32 per expander, yes.

And 4 of these 28 ports are used to connect the expander to the HBA/RAID AIC.

Link to the Intel documentation:

I’m 90 % sure this limit hadn’t been there back in 2017 when I purchased the first one :confused:

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